If a function declared nodiscard or a function returning an enumeration or class declared nodiscard by value is called from a discarded-value expression other than a cast to void, the compiler is encouraged to issue a warning.
[[nodiscard]] (1) (since C++17) [[nodiscard( string-literal )]] (2) (since C++20) string-literal - an unevaluated string literal that could be used to explain the rationale for why the result should not be discarded [edit] Explanation
Appears in a function declaration, enumeration declaration, or class declaration.
If, from a discarded-value expression other than a cast to void,
nodiscard is called, ornodiscard by value is called, ornodiscard is called by explicit type conversion or static_cast, ornodiscard is initialized by explicit type conversion or static_cast,the compiler is encouraged to issue a warning.
The string-literal, if specified, is usually included in the warnings.
(since C++20) [edit] Examplestruct [[nodiscard]] error_info { /*...*/ };
error_info enable_missile_safety_mode() { /*...*/ return {}; }
void launch_missiles() { /*...*/ }
void test_missiles()
{
enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
launch_missiles();
}
error_info& foo() { static error_info e; /*...*/ return e; }
void f1() { foo(); } // nodiscard type is not returned by value, no warning
// nodiscard( string-literal ) (since C++20):
[[nodiscard("PURE FUN")]] int strategic_value(int x, int y) { return x ^ y; }
int main()
{
strategic_value(4, 2); // compiler may warn on discarding a nodiscard value
auto z = strategic_value(0, 0); // OK: return value is not discarded
return z;
}
Possible output:
game.cpp:5:4: warning: ignoring return value of function declared with â® 'nodiscard' attribute game.cpp:17:5: warning: ignoring return value of function declared with â® 'nodiscard' attribute: PURE FUNStandard library
The following standard functions are declared with nodiscard attribute:
std::allocator<T>) [edit] allocate
[static]
allocates uninitialized storage using the allocatorstd::allocator_traits<Alloc>) [edit] allocate allocates memory
std::pmr::memory_resource) [edit] allocate allocate memory
std::pmr::polymorphic_allocator<T>) [edit] allocate allocates uninitialized storage using the outer allocator
std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>) [edit] Indirect access launder
(C++17)
pointer optimization barrier(C++20)
informs the compiler that a pointer is aligned(C++17)
checks whether the container is emptynode handle) empty checks whether the container is empty
std::array<T,N>) [edit] empty checks whether the string is empty
std::basic_string<CharT,Traits,Allocator>) [edit] empty checks whether the view is empty
std::basic_string_view<CharT,Traits>) [edit] empty checks whether the container is empty
std::deque<T,Allocator>) [edit] empty checks whether the container is empty
std::forward_list<T,Allocator>) [edit] empty checks whether the container is empty
std::list<T,Allocator>) [edit] empty checks whether the container is empty
std::map<Key,T,Compare,Allocator>) [edit] empty checks whether the match was successful
std::match_results<BidirIt,Alloc>) [edit] empty checks whether the container is empty
std::multimap<Key,T,Compare,Allocator>) [edit] empty checks whether the container is empty
std::multiset<Key,Compare,Allocator>) [edit] empty checks whether the container adaptor is empty
std::priority_queue<T,Container,Compare>) [edit] empty checks whether the container adaptor is empty
std::queue<T,Container>) [edit] empty checks whether the container is empty
std::set<Key,Compare,Allocator>) [edit] empty checks if the sequence is empty
std::span<T,Extent>) [edit] empty checks whether the container adaptor is empty
std::stack<T,Container>) [edit] empty checks whether the container is empty
std::unordered_map<Key,T,Hash,KeyEqual,Allocator>) [edit] empty checks whether the container is empty
std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>) [edit] empty checks whether the container is empty
std::unordered_multiset<Key,Hash,KeyEqual,Allocator>) [edit] empty checks whether the container is empty
std::unordered_set<Key,Hash,KeyEqual,Allocator>) [edit] empty checks whether the container is empty
std::vector<T,Allocator>) [edit] empty checks if the path is empty
std::filesystem::path) [edit] Miscellaneous async
(C++11)
runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the resultThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior P1771R1 C++17[[nodiscard]] on constructors has no effect can cause a warning if the constructed object is discarded [edit] References
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