CS 5006/5007 Style Guide

All header files should be self-contained. Users and refactoring tools should not have to adhere to special conditions to include the header. Specifically, a header should have header guards and include all other headers it needs.

Prefer placing the definitions for template and inline functions in the same file as their declarations. The definitions of these constructs must be included into every .cc file that uses them, or the program may fail to link in some build configurations. If declarations and definitions are in different files, including the former should transitively include the latter. Do not move these definitions to separately included header files (-inl.h); this practice was common in the past, but is no longer allowed.

As an exception, a template that is explicitly instantiated for all relevant sets of template arguments, or that is a private implementation detail of a class, is allowed to be defined in the one and only .cc file that instantiates the template.

There are rare cases where a file designed to be included is not self-contained. These are typically intended to be included at unusual locations, such as the middle of another file. They might not use header guards, and might not include their prerequisites. Name such files with the .inc extension. Use sparingly, and prefer self-contained headers when possible.

To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file foo/src/bar/baz.h in project foo should have the following guard:

#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_

...

#endif  // FOO_BAR_BAZ_H_

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, google-awesome-project/src/base/logging.h should be included as:

#include "base/logging.h"

In dir/foo.cc or dir/foo_test.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

1. dir2/foo2.h.
2. A blank line
3. C system files.
4. C++ system files.
5. A blank line
6. Other libraries' .h files.
7. Your project's .h files.

Note that any adjacent blank lines should be collapsed.

With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.

dir/foo.cc and dir2/foo2.h are usually in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but may sometimes be in different directories too.

Note that the C compatibility headers such as stddef.h are essentially interchangeable with their C++ counterparts (cstddef) Either style is acceptable, but prefer consistency with existing code.

Within each section the includes should be ordered alphabetically. Note that older code might not conform to this rule and should be fixed when convenient.

You should include all the headers that define the symbols you rely upon, except in the unusual case of forward declaration. If you rely on symbols from bar.h, don't count on the fact that you included foo.h which (currently) includes bar.h: include bar.h yourself, unless foo.h explicitly demonstrates its intent to provide you the symbols of bar.h. However, any includes present in the related header do not need to be included again in the related cc (i.e., foo.cc can rely on foo.h's includes).

For example, the includes in google-awesome-project/src/foo/internal/fooserver.cc might look like this:

#include "foo/server/fooserver.h"

#include <sys/types.h>
#include <unistd.h>
#include <vector>

#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/server/bar.h"

Sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized. Example:

#include "foo/public/fooserver.h"

#include "base/port.h"  // For LANG_CXX11.

#ifdef LANG_CXX11
#include <initializer_list>
#endif  // LANG_CXX11

C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g.:

int i;
i = f();      // Bad -- initialization separate from declaration.

int j = g();  // Good -- declaration has initialization.

std::vector<int> v;
v.push_back(1);  // Prefer initializing using brace initialization.
v.push_back(2);

std::vector<int> v = {1, 2};  // Good -- v starts initialized.

Variables needed for if, while and for statements should normally be declared within those statements, so that such variables are confined to those scopes. E.g.:

while (const char* p = strchr(str, '/')) str = p + 1;

There is one caveat: if the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope.

// Inefficient implementation:
for (int i = 0; i < 1000000; ++i) {
  Foo f;  // My ctor and dtor get called 1000000 times each.
  f.DoSomething(i);
}

It may be more efficient to declare such a variable used in a loop outside that loop:

Foo f;  // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i) {
  f.DoSomething(i);
}

const int kNum = 10;  // allowed

struct X { int n; };
const X kX[] = {{1}, {2}, {3}};  // allowed

void foo() {
  static const char* const kMessages[] = {"hello", "world"};  // allowed
}

// allowed: constexpr guarantees trivial destructor
constexpr std::array<int, 3> kArray = {{1, 2, 3}};

// bad: non-trivial destructor
const string kFoo = "foo";

// bad for the same reason, even though kBar is a reference (the
// rule also applies to lifetime-extended temporary objects)
const string& kBar = StrCat("a", "b", "c");

void bar() {
  // bad: non-trivial destructor
  static std::map<int, int> kData = {{1, 0}, {2, 0}, {3, 0}};
}

int n = 5;    // fine
int m = f();  // ? (depends on f)
Foo x;        // ? (depends on Foo::Foo)
Bar y = g();  // ? (depends on g and on Bar::Bar)

struct Foo { constexpr Foo(int) {} };

int n = 5;  // fine, 5 is a constant expression
Foo x(2);   // fine, 2 is a constant expression and the chosen constructor is constexpr
Foo a[] = { Foo(1), Foo(2), Foo(3) };  // fine

// Some declarations used below.
time_t time(time_t*);      // not constexpr!
int f();                   // not constexpr!
struct Bar { Bar() {} };

// Problematic initializations.
time_t m = time(nullptr);  // initializing expression not a constant expression
Foo y(f());                // ditto
Bar b;                     // chosen constructor Bar::Bar() not constexpr

int p = getpid();  // allowed, as long as no other static variable
                   // uses p in its own initialization

Common patterns

• Global strings: if you require a global or static string constant, consider using a simple character array, or a char pointer to the first element of a string literal. String literals have static storage duration already and are usually sufficient.
• Maps, sets, and other dynamic containers: if you require a static, fixed collection, such as a set to search against or a lookup table, you cannot use the dynamic containers from the standard library as a static variable, since they have non-trivial destructors. Instead, consider a simple array of trivial types, e.g. an array of arrays of ints (for a "map from int to int"), or an array of pairs (e.g. pairs of int and const char*). For small collections, linear search is entirely sufficient (and efficient, due to memory locality). If necessary, keep the collection in sorted order and use a binary search algorithm. If you do really prefer a dynamic container from the standard library, consider using a function-local static pointer, as described below.
• Smart pointers (unique_ptr, shared_ptr): smart pointers execute cleanup during destruction and are therefore forbidden. Consider whether your use case fits into one of the other patterns described in this section. One simple solution is to use a plain pointer to a dynamically allocated object and never delete it (see last item).
• Static variables of custom types: if you require static, constant data of a type that you need to define yourself, give the type a trivial destructor and a constexpr constructor.
• If all else fails, you can create an object dynamically and never delete it by binding the pointer to a function-local static pointer variable: static const auto* const impl = new T(args...); (If the initialization is more complex, it can be moved into a function or lambda expression.)

The output of a C++ function is naturally provided via a return value and sometimes via output parameters.

Prefer using return values instead of output parameters since they improve readability and oftentimes provide the same or better performance.

Parameters are either input to the function, output from the function, or both. Input parameters are usually values or const references, while output and input/output parameters will be pointers to non-const.

When ordering function parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the function just because they are new; place new input-only parameters before the output parameters.

This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with related functions may require you to bend the rule.

We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.

Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.

You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces.

Use 0 for integers and 0.0 for reals.

For pointers (address values), use nullptr, as this provides type-safety.

For C++03 projects, prefer NULL to 0. While the values are equivalent, NULL looks more like a pointer to the reader, and some C++ compilers provide special definitions of NULL which enable them to give useful warnings.

Use '\0' for the null character. Using the correct type makes the code more readable.

Use sizeof(varname) when you take the size of a particular variable. sizeof(varname) will update appropriately if someone changes the variable type either now or later. You may use sizeof(type) for code unrelated to any particular variable, such as code that manages an external or internal data format where a variable of an appropriate C++ type is not convenient.

Struct data;
memset(&data, 0, sizeof(data));

memset(&data, 0, sizeof(Struct));

if (raw_size < sizeof(int)) {
  LOG(ERROR) << "compressed record not big enough for count: " << raw_size;
  return false;
}

Give as descriptive a name as possible, within reason. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word. Abbreviations that would be familiar to someone outside your project with relevant domain knowledge are OK. As a rule of thumb, an abbreviation is probably OK if it's listed in Wikipedia.

int price_count_reader;    // No abbreviation.
int num_errors;            // "num" is a widespread convention.
int num_dns_connections;   // Most people know what "DNS" stands for.
int lstm_size;             // "LSTM" is a common machine learning abbreviation.

int n;                     // Meaningless.
int nerr;                  // Ambiguous abbreviation.
int n_comp_conns;          // Ambiguous abbreviation.
int wgc_connections;       // Only your group knows what this stands for.
int pc_reader;             // Lots of things can be abbreviated "pc".
int cstmr_id;              // Deletes internal letters.
FooBarRequestInfo fbri;    // Not even a word.

Note that certain universally-known abbreviations are OK, such as i for an iteration variable and T for a template parameter.

For some symbols, this style guide recommends names to start with a capital letter and to have a capital letter for each new word (a.k.a. "Camel Case" or "Pascal case"). When abbreviations or acronyms appear in such names, prefer to capitalize the abbreviations or acronyms as single words (i.e StartRpc(), not StartRPC()).

Template parameters should follow the naming style for their category: type template parameters should follow the rules for type names, and non-type template parameters should follow the rules for variable names.

Examples of acceptable file names:

• my_useful_class.cc
• my-useful-class.cc
• myusefulclass.cc
• myusefulclass_test.cc // _unittest and _regtest are deprecated.

C++ files should end in .cc and header files should end in .h. Files that rely on being textually included at specific points should end in .inc (see also the section on self-contained headers).

Do not use filenames that already exist in /usr/include, such as db.h.

In general, make your filenames very specific. For example, use http_server_logs.h rather than logs.h. A very common case is to have a pair of files called, e.g., foo_bar.h and foo_bar.cc, defining a class called FooBar.

The names of all types — classes, structs, type aliases, enums, and type template parameters — have the same naming convention. Type names should start with a capital letter and have a capital letter for each new word. No underscores. For example:

// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...

// typedefs
typedef hash_map<UrlTableProperties *, string> PropertiesMap;

// using aliases
using PropertiesMap = hash_map<UrlTableProperties *, string>;

// enums
enum UrlTableErrors { ...

Common Variable names

For example:

string table_name;  // OK - uses underscore.
string tablename;   // OK - all lowercase.

string tableName;   // Bad - mixed case.

Struct Data Members

Data members of structs, both static and non-static, are named like ordinary nonmember variables. They do not have the trailing underscores that data members in classes have.

struct UrlTableProperties {
  string name;
  int num_entries;
  static Pool<UrlTableProperties>* pool;
};

See Structs vs. Classes for a discussion of when to use a struct versus a class.

All such variables with static storage duration (i.e. statics and globals, see Storage Duration for details) should be named this way. This convention is optional for variables of other storage classes, e.g. automatic variables, otherwise the usual variable naming rules apply.

Ordinarily, functions should start with a capital letter and have a capital letter for each new word.

AddTableEntry()
DeleteUrl()
OpenFileOrDie()

(The same naming rule applies to class- and namespace-scope constants that are exposed as part of an API and that are intended to look like functions, because the fact that they're objects rather than functions is an unimportant implementation detail.)

Accessors and mutators (get and set functions) may be named like variables. These often correspond to actual member variables, but this is not required. For example, int count() and void set_count(int count).

The name of a top-level namespace should usually be the name of the project or team whose code is contained in that namespace. The code in that namespace should usually be in a directory whose basename matches the namespace name (or in subdirectories thereof).

Keep in mind that the rule against abbreviated names applies to namespaces just as much as variable names. Code inside the namespace seldom needs to mention the namespace name, so there's usually no particular need for abbreviation anyway.

Avoid nested namespaces that match well-known top-level namespaces. Collisions between namespace names can lead to surprising build breaks because of name lookup rules. In particular, do not create any nested std namespaces. Prefer unique project identifiers (websearch::index, websearch::index_util) over collision-prone names like websearch::util.

For internal namespaces, be wary of other code being added to the same internal namespace causing a collision (internal helpers within a team tend to be related and may lead to collisions). In such a situation, using the filename to make a unique internal name is helpful (websearch::index::frobber_internal for use in frobber.h)

Preferably, the individual enumerators should be named like constants. However, it is also acceptable to name them like macros. The enumeration name, UrlTableErrors (and AlternateUrlTableErrors), is a type, and therefore mixed case.

enum UrlTableErrors {
  kOK = 0,
  kErrorOutOfMemory,
  kErrorMalformedInput,
};
enum AlternateUrlTableErrors {
  OK = 0,
  OUT_OF_MEMORY = 1,
  MALFORMED_INPUT = 2,
};

Until January 2009, the style was to name enum values like macros. This caused problems with name collisions between enum values and macros. Hence, the change to prefer constant-style naming was put in place. New code should prefer constant-style naming if possible. However, there is no reason to change old code to use constant-style names, unless the old names are actually causing a compile-time problem.

Please see the description of macros; in general macros should not be used. However, if they are absolutely needed, then they should be named with all capitals and underscores.

#define ROUND(x) ...
#define PI_ROUNDED 3.0

bigopen()

function name, follows form of open()

uint

typedef

bigpos

struct or class, follows form of pos

sparse_hash_map

STL-like entity; follows STL naming conventions

LONGLONG_MAX

a constant, as in INT_MAX

You can use either the // or the /* */ syntax; however, // is much more common. Be consistent with how you comment and what style you use where.

Legal Notice and Author Line

Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL).

If you make significant changes to a file with an author line, consider deleting the author line. New files should usually not contain copyright notice or author line.

File Contents

If a .h declares multiple abstractions, the file-level comment should broadly describe the contents of the file, and how the abstractions are related. A 1 or 2 sentence file-level comment may be sufficient. The detailed documentation about individual abstractions belongs with those abstractions, not at the file level.

Do not duplicate comments in both the .h and the .cc. Duplicated comments diverge.

// Iterates over the contents of a GargantuanTable.
// Example:
//    GargantuanTableIterator* iter = table->NewIterator();
//    for (iter->Seek("foo"); !iter->done(); iter->Next()) {
//      process(iter->key(), iter->value());
//    }
//    delete iter;
class GargantuanTableIterator {
  ...
};

The class comment should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class. Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.

The class comment is often a good place for a small example code snippet demonstrating a simple and focused usage of the class.

When sufficiently separated (e.g. .h and .cc files), comments describing the use of the class should go together with its interface definition; comments about the class operation and implementation should accompany the implementation of the class's methods.

Function Declarations

Almost every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments may be omitted only if the function is simple and obvious (e.g. simple accessors for obvious properties of the class). These comments should be descriptive ("Opens the file") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments in the function definition.

Types of things to mention in comments at the function declaration:

• What the inputs and outputs are.
• For class member functions: whether the object remembers reference arguments beyond the duration of the method call, and whether it will free them or not.
• If the function allocates memory that the caller must free.
• Whether any of the arguments can be a null pointer.
• If there are any performance implications of how a function is used.
• If the function is re-entrant. What are its synchronization assumptions?

Here is an example:

// Returns an iterator for this table.  It is the client's
// responsibility to delete the iterator when it is done with it,
// and it must not use the iterator once the GargantuanTable object
// on which the iterator was created has been deleted.
//
// The iterator is initially positioned at the beginning of the table.
//
// This method is equivalent to:
//    Iterator* iter = table->NewIterator();
//    iter->Seek("");
//    return iter;
// If you are going to immediately seek to another place in the
// returned iterator, it will be faster to use NewIterator()
// and avoid the extra seek.
Iterator* GetIterator() const;

However, do not be unnecessarily verbose or state the completely obvious.

When documenting function overrides, focus on the specifics of the override itself, rather than repeating the comment from the overridden function. In many of these cases, the override needs no additional documentation and thus no comment is required.

When commenting constructors and destructors, remember that the person reading your code knows what constructors and destructors are for, so comments that just say something like "destroys this object" are not useful. Document what constructors do with their arguments (for example, if they take ownership of pointers), and what cleanup the destructor does. If this is trivial, just skip the comment. It is quite common for destructors not to have a header comment.

Function Definitions

If there is anything tricky about how a function does its job, the function definition should have an explanatory comment. For example, in the definition comment you might describe any coding tricks you use, give an overview of the steps you go through, or explain why you chose to implement the function in the way you did rather than using a viable alternative. For instance, you might mention why it must acquire a lock for the first half of the function but why it is not needed for the second half.

Note you should not just repeat the comments given with the function declaration, in the .h file or wherever. It's okay to recapitulate briefly what the function does, but the focus of the comments should be on how it does it.

Class Data Members

The purpose of each class data member (also called an instance variable or member variable) must be clear. If there are any invariants (special values, relationships between members, lifetime requirements) not clearly expressed by the type and name, they must be commented. However, if the type and name suffice (int num_events_;), no comment is needed.

In particular, add comments to describe the existence and meaning of sentinel values, such as nullptr or -1, when they are not obvious. For example:

private:
 // Used to bounds-check table accesses. -1 means
 // that we don't yet know how many entries the table has.
 int num_total_entries_;

Global Variables

All global variables should have a comment describing what they are, what they are used for, and (if unclear) why it needs to be global. For example:

// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;

Explanatory Comments

Tricky or complicated code blocks should have comments before them. Example:

// Divides result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i < result->size(); i++) {
  x = (x << 8) + (*result)[i];
  (*result)[i] = x >> 1;
  x &= 1;
}

Line Comments

Also, lines that are non-obvious should get a comment at the end of the line. These end-of-line comments should be separated from the code by 2 spaces. Example:

// If we have enough memory, mmap the data portion too.
mmap_budget = max<int64>(0, mmap_budget - index_->length());
if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
  return;  // Error already logged.

Note that there are both comments that describe what the code is doing, and comments that mention that an error has already been logged when the function returns.

If you have several comments on subsequent lines, it can often be more readable to line them up:

DoSomething();                  // Comment here so the comments line up.
DoSomethingElseThatIsLonger();  // Two spaces between the code and the comment.
{ // One space before comment when opening a new scope is allowed,
  // thus the comment lines up with the following comments and code.
  DoSomethingElse();  // Two spaces before line comments normally.
}
std::vector<string> list{
                    // Comments in braced lists describe the next element...
                    "First item",
                    // .. and should be aligned appropriately.
                    "Second item"};
DoSomething(); /* For trailing block comments, one space is fine. */

Function Argument Comments

When the meaning of a function argument is nonobvious, consider one of the following remedies:

• If the argument is a literal constant, and the same constant is used in multiple function calls in a way that tacitly assumes they're the same, you should use a named constant to make that constraint explicit, and to guarantee that it holds.
• Consider changing the function signature to replace a bool argument with an enum argument. This will make the argument values self-describing.
• For functions that have several configuration options, consider defining a single class or struct to hold all the options , and pass an instance of that. This approach has several advantages. Options are referenced by name at the call site, which clarifies their meaning. It also reduces function argument count, which makes function calls easier to read and write. As an added benefit, you don't have to change call sites when you add another option.
• Replace large or complex nested expressions with named variables.
• As a last resort, use comments to clarify argument meanings at the call site.

// What are these arguments?
const DecimalNumber product = CalculateProduct(values, 7, false, nullptr);

versus:

ProductOptions options;
options.set_precision_decimals(7);
options.set_use_cache(ProductOptions::kDontUseCache);
const DecimalNumber product =
    CalculateProduct(values, options, /*completion_callback=*/nullptr);

Don'ts

Do not state the obvious. In particular, don't literally describe what code does, unless the behavior is nonobvious to a reader who understands C++ well. Instead, provide higher level comments that describe why the code does what it does, or make the code self describing.

// Find the element in the vector.  <-- Bad: obvious!
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}

// Process "element" unless it was already processed.
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}

if (!IsAlreadyProcessed(element)) {
  Process(element);
}

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.

Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.

TODOs should include the string TODO in all caps, followed by the name, e-mail address, bug ID, or other identifier of the person or issue with the best context about the problem referenced by the TODO. The main purpose is to have a consistent TODO that can be searched to find out how to get more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO with a name, it is almost always your name that is given.

// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.
// TODO(bug 12345): remove the "Last visitors" feature

If your TODO is of the form "At a future date do something" make sure that you either include a very specific date ("Fix by November 2005") or a very specific event ("Remove this code when all clients can handle XML responses.").

You can mark an interface as deprecated by writing a comment containing the word DEPRECATED in all caps. The comment goes either before the declaration of the interface or on the same line as the declaration.

After the word DEPRECATED, write your name, e-mail address, or other identifier in parentheses.

A deprecation comment must include simple, clear directions for people to fix their callsites. In C++, you can implement a deprecated function as an inline function that calls the new interface point.

Marking an interface point DEPRECATED will not magically cause any callsites to change. If you want people to actually stop using the deprecated facility, you will have to fix the callsites yourself or recruit a crew to help you.

New code should not contain calls to deprecated interface points. Use the new interface point instead. If you cannot understand the directions, find the person who created the deprecation and ask them for help using the new interface point.

We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.

You shouldn't hard-code user-facing text in source, even English, so use of non-ASCII characters should be rare. However, in certain cases it is appropriate to include such words in your code. For example, if your code parses data files from foreign sources, it may be appropriate to hard-code the non-ASCII string(s) used in those data files as delimiters. More commonly, unittest code (which does not need to be localized) might contain non-ASCII strings. In such cases, you should use UTF-8, since that is an encoding understood by most tools able to handle more than just ASCII.

Hex encoding is also OK, and encouraged where it enhances readability — for example, "\xEF\xBB\xBF", or, even more simply, u8"\uFEFF", is the Unicode zero-width no-break space character, which would be invisible if included in the source as straight UTF-8.

Use the u8 prefix to guarantee that a string literal containing \uXXXX escape sequences is encoded as UTF-8. Do not use it for strings containing non-ASCII characters encoded as UTF-8, because that will produce incorrect output if the compiler does not interpret the source file as UTF-8.

You shouldn't use the C++11 char16_t and char32_t character types, since they're for non-UTF-8 text. For similar reasons you also shouldn't use wchar_t (unless you're writing code that interacts with the Windows API, which uses wchar_t extensively).

We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.

Functions look like this:

ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
  DoSomething();
  ...
}

If you have too much text to fit on one line:

ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
                                             Type par_name3) {
  DoSomething();
  ...
}

or if you cannot fit even the first parameter:

ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
    Type par_name1,  // 4 space indent
    Type par_name2,
    Type par_name3) {
  DoSomething();  // 2 space indent
  ...
}

Some points to note:

• Choose good parameter names.
• A parameter name may be omitted only if the parameter is not used in the function's definition.
• If you cannot fit the return type and the function name on a single line, break between them.
• If you break after the return type of a function declaration or definition, do not indent.
• The open parenthesis is always on the same line as the function name.
• There is never a space between the function name and the open parenthesis.
• There is never a space between the parentheses and the parameters.
• The open curly brace is always on the end of the last line of the function declaration, not the start of the next line.
• The close curly brace is either on the last line by itself or on the same line as the open curly brace.
• There should be a space between the close parenthesis and the open curly brace.
• All parameters should be aligned if possible.
• Default indentation is 2 spaces.
• Wrapped parameters have a 4 space indent.

Unused parameters that are obvious from context may be omitted:

class Foo {
 public:
  Foo(Foo&&);
  Foo(const Foo&);
  Foo& operator=(Foo&&);
  Foo& operator=(const Foo&);
};

Unused parameters that might not be obvious should comment out the variable name in the function definition:

class Shape {
 public:
  virtual void Rotate(double radians) = 0;
};

class Circle : public Shape {
 public:
  void Rotate(double radians) override;
};

void Circle::Rotate(double /*radians*/) {}

// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::Rotate(double) {}

Attributes, and macros that expand to attributes, appear at the very beginning of the function declaration or definition, before the return type:

MUST_USE_RESULT bool IsOK();

Function calls have the following format:

bool result = DoSomething(argument1, argument2, argument3);

If the arguments do not all fit on one line, they should be broken up onto multiple lines, with each subsequent line aligned with the first argument. Do not add spaces after the open paren or before the close paren:

bool result = DoSomething(averyveryveryverylongargument1,
                          argument2, argument3);

Arguments may optionally all be placed on subsequent lines with a four space indent:

if (...) {
  ...
  ...
  if (...) {
    bool result = DoSomething(
        argument1, argument2,  // 4 space indent
        argument3, argument4);
    ...
  }

Put multiple arguments on a single line to reduce the number of lines necessary for calling a function unless there is a specific readability problem. Some find that formatting with strictly one argument on each line is more readable and simplifies editing of the arguments. However, we prioritize for the reader over the ease of editing arguments, and most readability problems are better addressed with the following techniques.

If having multiple arguments in a single line decreases readability due to the complexity or confusing nature of the expressions that make up some arguments, try creating variables that capture those arguments in a descriptive name:

int my_heuristic = scores[x] * y + bases[x];
bool result = DoSomething(my_heuristic, x, y, z);

Or put the confusing argument on its own line with an explanatory comment:

bool result = DoSomething(scores[x] * y + bases[x],  // Score heuristic.
                          x, y, z);

If there is still a case where one argument is significantly more readable on its own line, then put it on its own line. The decision should be specific to the argument which is made more readable rather than a general policy.

Sometimes arguments form a structure that is important for readability. In those cases, feel free to format the arguments according to that structure:

// Transform the widget by a 3x3 matrix.
my_widget.Transform(x1, x2, x3,
                    y1, y2, y3,
                    z1, z2, z3);

If the braced list follows a name (e.g. a type or variable name), format as if the {} were the parentheses of a function call with that name. If there is no name, assume a zero-length name.

// Examples of braced init list on a single line.
return {foo, bar};
functioncall({foo, bar});
std::pair<int, int> p{foo, bar};

// When you have to wrap.
SomeFunction(
    {"assume a zero-length name before {"},
    some_other_function_parameter);
SomeType variable{
    some, other, values,
    {"assume a zero-length name before {"},
    SomeOtherType{
        "Very long string requiring the surrounding breaks.",
        some, other values},
    SomeOtherType{"Slightly shorter string",
                  some, other, values}};
SomeType variable{
    "This is too long to fit all in one line"};
MyType m = {  // Here, you could also break before {.
    superlongvariablename1,
    superlongvariablename2,
    {short, interior, list},
    {interiorwrappinglist,
     interiorwrappinglist2}};

There are two acceptable formats for a basic conditional statement. One includes spaces between the parentheses and the condition, and one does not.

The most common form is without spaces. Either is fine, but be consistent. If you are modifying a file, use the format that is already present. If you are writing new code, use the format that the other files in that directory or project use. If in doubt and you have no personal preference, do not add the spaces.

if (condition) {  // no spaces inside parentheses
  ...  // 2 space indent.
} else if (...) {  // The else goes on the same line as the closing brace.
  ...
} else {
  ...
}

If you prefer you may add spaces inside the parentheses:

if ( condition ) {  // spaces inside parentheses - rare
  ...  // 2 space indent.
} else {  // The else goes on the same line as the closing brace.
  ...
}

Note that in all cases you must have a space between the if and the open parenthesis. You must also have a space between the close parenthesis and the curly brace, if you're using one.

if(condition) {   // Bad - space missing after IF.
if (condition){   // Bad - space missing before {.
if(condition){    // Doubly bad.

if (condition) {  // Good - proper space after IF and before {.

Short conditional statements may be written on one line if this enhances readability. You may use this only when the line is brief and the statement does not use the else clause.

if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();

This is not allowed when the if statement has an else:

// Not allowed - IF statement on one line when there is an ELSE clause
if (x) DoThis();
else DoThat();

In general, curly braces are not required for single-line statements, but they are allowed if you like them; conditional or loop statements with complex conditions or statements may be more readable with curly braces. Some projects require that an if must always have an accompanying brace.

if (condition)
  DoSomething();  // 2 space indent.

if (condition) {
  DoSomething();  // 2 space indent.
}

However, if one part of an if-else statement uses curly braces, the other part must too:

// Not allowed - curly on IF but not ELSE
if (condition) {
  foo;
} else
  bar;

// Not allowed - curly on ELSE but not IF
if (condition)
  foo;
else {
  bar;
}

// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
  foo;
} else {
  bar;
}

case blocks in switch statements can have curly braces or not, depending on your preference. If you do include curly braces they should be placed as shown below.

If not conditional on an enumerated value, switch statements should always have a default case (in the case of an enumerated value, the compiler will warn you if any values are not handled). If the default case should never execute, treat this as an error. For example:

switch (var) {
  case 0: {  // 2 space indent
    ...      // 4 space indent
    break;
  }
  case 1: {
    ...
    break;
  }
  default: {
    assert(false);
  }
}

Fall-through from one case label to another must be annotated using the ABSL_FALLTHROUGH_INTENDED; macro (defined in absl/base/macros.h). ABSL_FALLTHROUGH_INTENDED; should be placed at a point of execution where a fall-through to the next case label occurs. A common exception is consecutive case labels without intervening code, in which case no annotation is needed.

switch (x) {
  case 41:  // No annotation needed here.
  case 43:
    if (dont_be_picky) {
      // Use this instead of or along with annotations in comments.
      ABSL_FALLTHROUGH_INTENDED;
    } else {
      CloseButNoCigar();
      break;
    }
  case 42:
    DoSomethingSpecial();
    ABSL_FALLTHROUGH_INTENDED;
  default:
    DoSomethingGeneric();
    break;
}

Braces are optional for single-statement loops.

for (int i = 0; i < kSomeNumber; ++i)
  printf("I love you\n");

for (int i = 0; i < kSomeNumber; ++i) {
  printf("I take it back\n");
}

Empty loop bodies should use either an empty pair of braces or continue with no braces, rather than a single semicolon.

while (condition) {
  // Repeat test until it returns false.
}
for (int i = 0; i < kSomeNumber; ++i) {}  // Good - one newline is also OK.
while (condition) continue;  // Good - continue indicates no logic.

while (condition);  // Bad - looks like part of do/while loop.

The following are examples of correctly-formatted pointer and reference expressions:

x = *p;
p = &x;
x = r.y;
x = r->y;

Note that:

• There are no spaces around the period or arrow when accessing a member.
• Pointer operators have no space after the * or &.

When declaring a pointer variable or argument, you may place the asterisk adjacent to either the type or to the variable name:

// These are fine, space preceding.
char *c;
const string &str;

// These are fine, space following.
char* c;
const string& str;

You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.

// Fine if helpful for readability.
int x, y;

int x, *y;  // Disallowed - no & or * in multiple declaration
char * c;  // Bad - spaces on both sides of *
const string & str;  // Bad - spaces on both sides of &

In this example, the logical AND operator is always at the end of the lines:

if (this_one_thing > this_other_thing &&
    a_third_thing == a_fourth_thing &&
    yet_another && last_one) {
  ...
}

Note that when the code wraps in this example, both of the && logical AND operators are at the end of the line. This is more common in Google code, though wrapping all operators at the beginning of the line is also allowed. Feel free to insert extra parentheses judiciously because they can be very helpful in increasing readability when used appropriately. Also note that you should always use the punctuation operators, such as && and ~, rather than the word operators, such as and and compl.

Use parentheses in return expr; only where you would use them in x = expr;.

return result;                  // No parentheses in the simple case.
// Parentheses OK to make a complex expression more readable.
return (some_long_condition &&
        another_condition);

return (value);                // You wouldn't write var = (value);
return(result);                // return is not a function!

You may choose between =, (), and {}; the following are all correct:

int x = 3;
int x(3);
int x{3};
string name = "Some Name";
string name("Some Name");
string name{"Some Name"};

Be careful when using a braced initialization list {...} on a type with an std::initializer_list constructor. A nonempty braced-init-list prefers the std::initializer_list constructor whenever possible. Note that empty braces {} are special, and will call a default constructor if available. To force the non-std::initializer_list constructor, use parentheses instead of braces.

std::vector<int> v(100, 1);  // A vector containing 100 items: All 1s.
std::vector<int> v{100, 1};  // A vector containing 2 items: 100 and 1.

Also, the brace form prevents narrowing of integral types. This can prevent some types of programming errors.

int pi(3.14);  // OK -- pi == 3.
int pi{3.14};  // Compile error: narrowing conversion.

Even when preprocessor directives are within the body of indented code, the directives should start at the beginning of the line.

// Good - directives at beginning of line
  if (lopsided_score) {
#if DISASTER_PENDING      // Correct -- Starts at beginning of line
    DropEverything();
# if NOTIFY               // OK but not required -- Spaces after #
    NotifyClient();
# endif
#endif
    BackToNormal();
  }

// Bad - indented directives
  if (lopsided_score) {
    #if DISASTER_PENDING  // Wrong!  The "#if" should be at beginning of line
    DropEverything();
    #endif                // Wrong!  Do not indent "#endif"
    BackToNormal();
  }

The basic format for a class definition (lacking the comments, see Class Comments for a discussion of what comments are needed) is:

class MyClass : public OtherClass {
 public:      // Note the 1 space indent!
  MyClass();  // Regular 2 space indent.
  explicit MyClass(int var);
  ~MyClass() {}

  void SomeFunction();
  void SomeFunctionThatDoesNothing() {
  }

  void set_some_var(int var) { some_var_ = var; }
  int some_var() const { return some_var_; }

 private:
  bool SomeInternalFunction();

  int some_var_;
  int some_other_var_;
};

Things to note:

• Any base class name should be on the same line as the subclass name, subject to the 80-column limit.
• The public:, protected:, and private: keywords should be indented one space.
• Except for the first instance, these keywords should be preceded by a blank line. This rule is optional in small classes.
• Do not leave a blank line after these keywords.
• The public section should be first, followed by the protected and finally the private section.
• See Declaration Order for rules on ordering declarations within each of these sections.

The acceptable formats for initializer lists are:

// When everything fits on one line:
MyClass::MyClass(int var) : some_var_(var) {
  DoSomething();
}

// If the signature and initializer list are not all on one line,
// you must wrap before the colon and indent 4 spaces:
MyClass::MyClass(int var)
    : some_var_(var), some_other_var_(var + 1) {
  DoSomething();
}

// When the list spans multiple lines, put each member on its own line
// and align them:
MyClass::MyClass(int var)
    : some_var_(var),             // 4 space indent
      some_other_var_(var + 1) {  // lined up
  DoSomething();
}

// As with any other code block, the close curly can be on the same
// line as the open curly, if it fits.
MyClass::MyClass(int var)
    : some_var_(var) {}

Namespaces do not add an extra level of indentation. For example, use:

namespace {

void foo() {  // Correct.  No extra indentation within namespace.
  ...
}

}  // namespace

Do not indent within a namespace:

namespace {

  // Wrong!  Indented when it should not be.
  void foo() {
    ...
  }

}  // namespace

When declaring nested namespaces, put each namespace on its own line.

namespace foo {
namespace bar {

General

void f(bool b) {  // Open braces should always have a space before them.
  ...
int i = 0;  // Semicolons usually have no space before them.
// Spaces inside braces for braced-init-list are optional.  If you use them,
// put them on both sides!
int x[] = { 0 };
int x[] = {0};

// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar {
 public:
  // For inline function implementations, put spaces between the braces
  // and the implementation itself.
  Foo(int b) : Bar(), baz_(b) {}  // No spaces inside empty braces.
  void Reset() { baz_ = 0; }  // Spaces separating braces from implementation.
  ...

Adding trailing whitespace can cause extra work for others editing the same file, when they merge, as can removing existing trailing whitespace. So: Don't introduce trailing whitespace. Remove it if you're already changing that line, or do it in a separate clean-up operation (preferably when no-one else is working on the file).

Loops and Conditionals

if (b) {          // Space after the keyword in conditions and loops.
} else {          // Spaces around else.
}
while (test) {}   // There is usually no space inside parentheses.
switch (i) {
for (int i = 0; i < 5; ++i) {
// Loops and conditions may have spaces inside parentheses, but this
// is rare.  Be consistent.
switch ( i ) {
if ( test ) {
for ( int i = 0; i < 5; ++i ) {
// For loops always have a space after the semicolon.  They may have a space
// before the semicolon, but this is rare.
for ( ; i < 5 ; ++i) {
  ...

// Range-based for loops always have a space before and after the colon.
for (auto x : counts) {
  ...
}
switch (i) {
  case 1:         // No space before colon in a switch case.
    ...
  case 2: break;  // Use a space after a colon if there's code after it.

Operators

// Assignment operators always have spaces around them.
x = 0;

// Other binary operators usually have spaces around them, but it's
// OK to remove spaces around factors.  Parentheses should have no
// internal padding.
v = w * x + y / z;
v = w*x + y/z;
v = w * (x + z);

// No spaces separating unary operators and their arguments.
x = -5;
++x;
if (x && !y)
  ...

Vertical Whitespace

Exceptions to the Rules

The coding conventions described above are mandatory. However, like all good rules, these sometimes have exceptions, which we discuss here.

Parting Words