Attributes C++11 Attributes C++11 Attributes

    Basic Syntax

    [[attribute_name]]
    [[attribute_name(arguments…)]]
    [[attrib_namespace::attribute_name]]

    [[using attrib_namespace: attrib1, attrib2,…]]  C++17

    [[nodiscard]] C++17
    [[nodiscard("why")]]  C++20
    [[nodiscard("because...")]] ReturnType function () { … } auto x = function(); //  function(); //  COMPILER WARNING because...
    enum [[nodiscard]] E { a, b }; E make_E() { return E::a; } auto e = make_E(); //  make_E(); //  COMPILER WARNING
    class [[nodiscard]] C { … } C make_C() { return C{}; } auto c = make_C(); //  make_C(); //  COMPILER WARNING
    class A { [[nodiscard]] A (int) {…} … }; A a {1}; //  A{0}; //  COMPILER WARNING

    encourages compilers to issue warnings if (return) values are discarded

    [[deprecated]]
    [[deprecated("why")]]

    C++14

    [[deprecated]] void old_function (); namespace [[deprecated]] v1 { … } using OldAlias [[deprecated]] = SomeType; enum [[deprecated]] Enumeration {}; enum E { enumerator [[deprecated]], enumerator2 [[deprecated]] = 7 }; struct S { [[deprecated]] Member m_; }; class [[deprecated("use D instead")]] C { … } template<typename T> class X; template<> class X<double> {}; template<> class [[deprecated]] X<float> {};
    • indicates that an entity should no longer be used
    • compilers usually issue warnings on usage
    [[fallthrough]]

    C++17

    switch (expression) { case 1: handleA(); case 2: [[fallthrough]]; case 3: handleB(); default: handleC(); };
    • indicates that fall through from a previous case label is intentional and should not be diagnosed
    • can only be applied to a null statement
    [[likely]] [[unlikely]] [[likely]]

    C++20

    if (…) { [[likely]] … } else { [[unlikely]] … } switch (…) { [[likely]] case 0: foo(); break; case 1: bar(); break; [[unlikely]] case 2: baz(); break; }

    allows compilers to optimize for cases where the path of execution that includes a statement or label marked [[likely]] / [[unlikely]] is more/less likely

    [[maybe_unused]]

    C++17

    [[maybe_unused]] void function (); using Alias [[maybe_unused]] = SomeType; enum [[maybe_unused]] Enumeration {}; enum E { enumerator [[maybe_unused]], enumerator2 [[maybe_unused]] = 7 }; struct S { [[maybe_unused]] Member m_; }; class [[maybe_unused]] C { … }

    suppresses compiler diagnostics about unused functions, types or enumerators

    [[noreturn]]

    C++11

    [[noreturn]] void always_throwing (…) { … // no return statement! if (…) throw Exception_Type1{}; … // no return statement! throw Exception_Type2{}; } // never reached!
    • specifies that a function never returns, i.e., it always throws an exception
    • if [[noreturn]] function actually returns ⇒ undefined behavior
    [[no_unique_address]]

    C++20

    struct EmptyType {}; struct S { Type member1_; [[no_unique_address]] EmptyType member2_ {}; };
    • may be applied to non-static, non-bitfield members
    • indicates that a member need not have an address that is distinct from all other non-static data members
    • if a [[no_unique_address]] member is non-empty any trailing padding may be reused to store other members
    [[carries_dependency]]

    C++11

    Allows compilers to omit memory fences in release-consume memory order if a dependency chain propagates in or out of a function:

    void unknown_dependency (Type*); void propagates (Type* [[carries_dependency]]); std::atomic<Type*> atm; Type* p = atm.load(std::memory_order_consume); if (p) p->load(); // propagates if (p) unknown_dependency (p); // compiler might introduce memory fence // if function definition 'invisible' if (p) propagates (p); // compiler can omit memory fence!

    specify that a parameter carries a dependency into a function/lambda: 

    void function (Param [[carries_dependency]] p) { … } auto f = [](Param [[carries_dependency]] p) { … }

    specify that the return value carries a dependency out of the function:

    [[carries_dependency]] ReturnType function (Param p) { … }

    Related …

    Compiler References

    C++ Weekly Episodes

    Last updated: 2021-03-28

    Comments…