# 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]]` |  |

### 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

operator new operator 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 underlying container is empty (public member function of `std::priority_queue<T,Container,Compare>`)
empty | checks whether the underlying container 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 underlying container 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)

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