C++ Attributes

C++11 introduced standard attributes: a way to mark fragments of code with useful information for the developer or optimization information for the compiler. See a complete list of standard attributes here, Clang attributes here, and Microsoft attributes here. I will go over a few of them in this post.

• [[nodiscard]]

  – when specified with a function declaration, tells the compiler to emit a warning if function’s return value is ignored; when specified with a struct emits a warning wherever the struct is returned and ignored.

• [[fallthrough]]

  – suppresses compiler warning about a switch-case statement without a

  break

  ; in other words, a

  case

  statement that falls through into another

  case

  .

• [[no_unique_address]]

  – tells the compiler to perform empty base optimization on marked data member.

• [[deprecated]]

  – emits a warning when marked function, struct, namespace, or variable is used.

• [[noreturn]]

  – tells the compiler that the marked function never returns; emits a warning if it does.

• [[maybe_unused]]

  – suppressed a warning when marked variable, function, or argument is not used.

• [[likely]]

  – tell the compiler this is the most likely path of execution; allows for optimizations.

Note: not all are supported on every compiler; I tested on LLVM 8.0.0 and GCC 8.2. Luckily the unsupported ones do not cause a compile error 🙂

Below is an example code and a screenshot of compiler messages.

attributes.cpp:

[[nodiscard]] int no_discard(int i)
{
	switch(i)
	{
		case 1:
		[[fallthrough]];
		[[likely]] case 2: return i;
		default: return i;
	}
}

struct E {};

struct [[nodiscard]] S
{
	int i;
	[[no_unique_address]] E e;
};

S no_discard_struct()
{
	return {};
}

struct [[deprecated("Use Q instead")]] P {};

[[deprecated("Use bar() instead")]] void foo() {}

[[noreturn]] void never_returns()
{
	while(true);
}

int main()
{
	[[maybe_unused]] int unused = 1;
	P p;

	no_discard(0);
	no_discard_struct();
	foo();
	never_returns();
}

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