C++ attribute: nodiscard (since C++17)

 

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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.

Syntax

[[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

Explanation

Appears in a function declaration, enumeration declaration, or class declaration.

If, from a discarded-value expression other than a cast to void,

• a function declared nodiscard is called, or
• a function returning an enumeration or class declared nodiscard by value is called, or
• a constructor declared nodiscard is called by explicit type conversion or static_cast, or
• an object of an enumeration or class type declared nodiscard 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)

Example

Run this code

struct [[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 ⮠
 'nodiscard' attribute
game.cpp:17:5: warning: ignoring return value of function declared with ⮠
 'nodiscard' attribute: PURE FUN

Standard library The following standard functions are declared with nodiscard attribute: Extended content Allocation functions operator newoperator new[] allocation functions (function) allocate allocates uninitialized storage (public member function of std::allocator<T>) allocate [static] allocates uninitialized storage using the allocator (public static member function of std::allocator_traits<Alloc>) allocate allocates memory (public member function of std::pmr::memory_resource) allocate allocate memory (public member function of std::pmr::polymorphic_allocator<T>) allocate allocates uninitialized storage using the outer allocator (public member function of std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>) Indirect access launder (C++17) pointer optimization barrier (function template) assume_aligned (C++20) informs the compiler that a pointer is aligned (function template) Emptiness-checking functions empty (C++17) checks whether the container is empty (function template) empty checks whether the node handle is empty (public member function of node handle) empty checks whether the container is empty (public member function of std::array<T,N>) empty checks whether the string is empty (public member function of std::basic_string<CharT,Traits,Allocator>) empty checks whether the view is empty (public member function of std::basic_string_view<CharT,Traits>) empty checks whether the container is empty (public member function of std::deque<T,Allocator>) empty checks whether the container is empty (public member function of std::forward_list<T,Allocator>) empty checks whether the container is empty (public member function of std::list<T,Allocator>) empty checks whether the container is empty (public member function of std::map<Key,T,Compare,Allocator>) empty checks whether the match was successful (public member function of std::match_results<BidirIt,Alloc>) empty checks whether the container is empty (public member function of std::multimap<Key,T,Compare,Allocator>) empty checks whether the container is empty (public member function of std::multiset<Key,Compare,Allocator>) empty checks whether the container adaptor is empty (public member function of std::priority_queue<T,Container,Compare>) empty checks whether the container adaptor is empty (public member function of std::queue<T,Container>) empty checks whether the container is empty (public member function of std::set<Key,Compare,Allocator>) empty checks if the sequence is empty (public member function of std::span<T,Extent>) empty checks whether the container adaptor is empty (public member function of std::stack<T,Container>) empty checks whether the container is empty (public member function of std::unordered_map<Key,T,Hash,KeyEqual,Allocator>) empty checks whether the container is empty (public member function of std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>) empty checks whether the container is empty (public member function of std::unordered_multiset<Key,Hash,KeyEqual,Allocator>) empty checks whether the container is empty (public member function of std::unordered_set<Key,Hash,KeyEqual,Allocator>) empty checks whether the container is empty (public member function of std::vector<T,Allocator>) empty checks if the path is empty (public member function of std::filesystem::path) Miscellaneous async (C++11) runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)

(until C++26)

Defect reports

The 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

References

• C++23 standard (ISO/IEC 14882:2024):

• 9.12.9 Nodiscard attribute [dcl.attr.nodiscard]

• C++20 standard (ISO/IEC 14882:2020):

• 9.12.8 Nodiscard attribute [dcl.attr.nodiscard]

• C++17 standard (ISO/IEC 14882:2017):

• 10.6.7 Nodiscard attribute [dcl.attr.nodiscard]

See also

ignore (C++11)	placeholder to skip an element when unpacking a tuple using tie (constant)
C documentation for nodiscard

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