Unlike the style guide, the content of this page is advisory, not required. You can always deviate from something on this page, if the relevant author/reviewer/OWNERS agree that another course is better.
Dynamic initialization of function-scope static variables is thread-safe in Chromium (per standard C++11 behavior). Before 2017, this was thread-unsafe, and base::LazyInstance was widely used. This is no longer necessary. Background can be found in this thread and this thread.
void foo() { static int ok_count = ComputeTheCount(); // OK; a problem pre-2017. static int good_count = 42; // Done before dynamic initialization. static constexpr int better_count = 42; // Even better (likely inlined at compile time). static auto& object = *new Object; // For class types. }
The Google Style Guide says classes can omit copy/move declarations or deletions “only if they are obvious”. Because “obvious” is subjective and even the examples in the style guide take some thought to figure out, being explicit is clear, simple, and avoids any risk of accidental copying.
Declare or delete these operations in the public section, between other constructors and the destructor; DISALLOW_COPY_AND_ASSIGN
is deprecated. For a non-copyable/movable type, delete the copy operations (the move operations will be implicitly deleted); otherwise, declare either copy operations, move operations, or both (a non-declared pair will be implicitly deleted). Always declare or delete both construction and assignment, not just one (which can introduce subtle bugs).
There are myriad ways to initialize variables in C++11. Prefer the following general rules:
Use assignment syntax when performing “simple” initialization with one or more literal values which will simply be composed into the object:
int i = 1; std::string s = "Hello"; std::pair<bool, double> p = {true, 2.0}; std::vector<std::string> v = {"one", "two", "three"};
Using ‘=’ here is no less efficient than “()” (the compiler won‘t generate a temp + copy), and ensures that only implicit constructors are called, so readers seeing this syntax can assume nothing complex or subtle is happening. Note that “{}” are allowed on the right side of the ‘=’ here (e.g. when you’re merely passing a set of initial values to a “simple” struct/ container constructor; see below items for contrast).
Use constructor syntax when construction performs significant logic, uses an explicit constructor, or in some other way is not intuitively “simple” to the reader:
MyClass c(1.7, false, "test"); std::vector<double> v(500, 0.97); // Creates 50 copies of the provided initializer
Use C++11 “uniform init” syntax (“{}” without ‘=’) only when neither of the above work:
class C { public: explicit C(bool b) { ... }; ... }; class UsesC { ... private: C c{true}; // Cannot use '=' since C() is explicit (and "()" is invalid syntax here) }; class Vexing { public: explicit Vexing(const std::string& s) { ... }; ... }; void func() { Vexing v{std::string()}; // Using "()" here triggers "most vexing parse"; // "{}" is arguably more readable than "(())" ... }
Never mix uniform init syntax with auto, since what it deduces is unlikely to be what was intended:
auto x{1}; // Until C++17, decltype(x) is std::initializer_list<int>, not int!
If possible, initialize class members in their declarations, except where a member's value is explicitly set by every constructor.
This reduces the chance of uninitialized variables, documents default values in the declaration, and increases the number of constructors that can use =default
(see below).
class C { public: C() : a_(2) {} C(int b) : a_(1), b_(b) {} private: int a_; // Not necessary to init this since all constructors set it. int b_ = 0; // Not all constructors set this. std::string c_; // No initializer needed due to string's default constructor. base::WeakPtrFactory<C> factory_{this}; // {} allows calling of explicit constructors. };
Note that it's possible to call functions or pass this
and other expressions in initializers, so even some complex initializations can be done in the declaration.
std::make_unique
and base::MakeRefCounted
instead of bare new
When possible, avoid bare new
by using std::make_unique<T>(...)
and base::MakeRefCounted<T>(...)
:
// BAD: bare call to new; for refcounted types, not compatible with one-based // refcounting. return base::WrapUnique(new T(1, 2, 3)); return base::WrapRefCounted(new T(1, 2, 3)); // BAD: same as the above, plus mentions type names twice. std::unique_ptr<T> t(new T(1, 2, 3)); base::scoped_refptr<T> t(new T(1, 2, 3)); return std::unique_ptr<T>(new T(1, 2, 3)); return base::scoped_refptr<T>(new T(1, 2, 3)); // OK, but verbose: type name still mentioned twice. std::unique_ptr<T> t = std::make_unique<T>(1, 2, 3); base::scoped_refptr<T> t = base::MakeRefCounted<T>(1, 2, 3); // GOOD; make_unique<>/MakeRefCounted<> are clear enough indicators of the // returned type. auto t = std::make_unique<T>(1, 2, 3); auto t = base::MakeRefCounted<T>(1, 2, 3); return std::make_unique<T>(1, 2, 3); return base::MakeRefCounted<T>(1, 2, 3);
Notes:
Never friend std::make_unique
to work around constructor access restrictions. It will allow anyone to construct the class. Use base::WrapUnique
in this case.
DON'T:
class Bad { public: std::unique_ptr<Bad> Create() { return std::make_unique<Bad>(); } // ... private: Bad(); // ... friend std::unique_ptr<Bad> std::make_unique<Bad>(); // Lost access control };
DO:
class Okay { public: // For explanatory purposes. If Create() adds no value, it is better just // to have a public constructor instead. std::unique_ptr<Okay> Create() { return base::WrapUnique(new Okay()); } // ... private: Okay(); // ... };
base::WrapUnique(new Foo)
and base::WrapUnique(new Foo())
mean something different if Foo
does not have a user-defined constructor. Don‘t make future maintainers guess whether you left off the ‘()’ on purpose. Use std::make_unique<Foo>()
instead. If you’re intentionally leaving off the “()” as an optimization, please leave a comment.
auto a = base::WrapUnique(new A); // BAD: "()" omitted intentionally? auto a = std::make_unique<A>(); // GOOD // "()" intentionally omitted to avoid unnecessary zero-initialization. // base::WrapUnique() does the wrong thing for array pointers. auto array = std::unique_ptr<A[]>(new A[size]);
See also TOTW 126.
auto
to deduce a raw pointerDo not use auto
when the type would be deduced to be a pointer type; this can cause confusion. Instead, specify the “pointer” part outside of auto
:
auto item = new Item(); // BAD: auto deduces to Item*, type of |item| is Item* auto* item = new Item(); // GOOD: auto deduces to Item, type of |item| is Item*
const
correctlyFor safety and simplicity, don't return pointers or references to non-const objects from const methods. Within that constraint, mark methods as const where possible. Avoid const_cast
to remove const, except when implementing non-const getters in terms of const getters.
For more information, see Using Const Correctly.
=default
Use =default
to define special member functions where possible, even if the default implementation is just {}. Be careful when defaulting move operations. Moved-from objects must be in a valid but unspecified state, i.e., they must satisfy the class invariants, and the default implementations may not achieve this.
class Good { public: // We can, and usually should, provide the default implementation separately // from the declaration. Good(); // Use =default here for consistency, even though the implementation is {}. ~Good() = default; Good(const Good& other) = default; private: std::vector<int> v_; }; Good::Good() = default;
The common ways to represent names in comments are as follows:
FooClass
FooFunction()
. The trailing parens disambiguate against class names, and, occasionally, English words.|foo_var|
. Again, the vertical lines disambiguate against English words, and, occasionally, inlined function names. Code search will also automatically convert |foo_var|
into a clickable link.// FooImpl implements the FooBase class. // FooFunction() modifies |foo_member_|.
Named namespaces are discouraged in top-level embedders (e.g., chrome/
). See this thread for background and discussion.