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