Home Original page

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

From cppreference.com

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,

the compiler is encouraged to issue a warning.

The string-literal, if specified, is usually included in the warnings.

(since C++20)

Example

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
allocation functions
(function) [edit]
allocates uninitialized storage
(public member function of std::allocator<T>) [edit]

[static]

 allocates uninitialized storage using the allocator
(public static member function of std::allocator_traits<Alloc>) [edit]
allocates memory
(public member function of std::pmr::memory_resource) [edit]
allocate memory
(public member function of std::pmr::polymorphic_allocator<T>) [edit]
allocates uninitialized storage using the outer allocator
(public member function of std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>) [edit]
Indirect access

(C++17)

 pointer optimization barrier
(function template) [edit]
informs the compiler that a pointer is aligned
(function template) [edit]
Emptiness-checking functions

(C++17)

 checks whether the container is empty
(function template) [edit]
checks whether the node handle is empty
(public member function of node handle)
checks whether the container is empty
(public member function of std::array<T,N>) [edit]
checks whether the string is empty
(public member function of std::basic_string<CharT,Traits,Allocator>) [edit]
checks whether the view is empty
(public member function of std::basic_string_view<CharT,Traits>) [edit]
checks whether the container is empty
(public member function of std::deque<T,Allocator>) [edit]
checks whether the container is empty
(public member function of std::forward_list<T,Allocator>) [edit]
checks whether the container is empty
(public member function of std::list<T,Allocator>) [edit]
checks whether the container is empty
(public member function of std::map<Key,T,Compare,Allocator>) [edit]
checks whether the match was successful
(public member function of std::match_results<BidirIt,Alloc>) [edit]
checks whether the container is empty
(public member function of std::multimap<Key,T,Compare,Allocator>) [edit]
checks whether the container is empty
(public member function of std::multiset<Key,Compare,Allocator>) [edit]
checks whether the container adaptor is empty
(public member function of std::priority_queue<T,Container,Compare>) [edit]
checks whether the container adaptor is empty
(public member function of std::queue<T,Container>) [edit]
checks whether the container is empty
(public member function of std::set<Key,Compare,Allocator>) [edit]
checks if the sequence is empty
(public member function of std::span<T,Extent>) [edit]
checks whether the container adaptor is empty
(public member function of std::stack<T,Container>) [edit]
checks whether the container is empty
(public member function of std::unordered_map<Key,T,Hash,KeyEqual,Allocator>) [edit]
checks whether the container is empty
(public member function of std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>) [edit]
checks whether the container is empty
(public member function of std::unordered_multiset<Key,Hash,KeyEqual,Allocator>) [edit]
checks whether the container is empty
(public member function of std::unordered_set<Key,Hash,KeyEqual,Allocator>) [edit]
checks whether the container is empty
(public member function of std::vector<T,Allocator>) [edit]
checks if the path is empty
(public member function of std::filesystem::path) [edit]
Miscellaneous

(C++11)

 runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result
(function template) [edit]
(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