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

From cppreference.com
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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.

Contents

[edit] Syntax

[[nodiscard]] (1)
[[nodiscard( string-literal )]] (2) (since C++20)
string-literal - text 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,

the compiler is encouraged to issue a warning.

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

(since C++20)

[edit] Example

struct [[nodiscard]] error_info { };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
   enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
   launch_missiles();
}
error_info& foo();
void f1() {
    foo(); // nodiscard type is not returned by value, no warning
}


[edit] 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

[edit] Standard library

The following standard functions are declared with nodiscard attribute:

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 underlying container is empty
(public member function of std::priority_queue<T,Container,Compare>) [edit]
checks whether the underlying container 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 underlying container 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]