哋它亢++ attribute: nodiscard (since 哋它亢++17)

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

哋它亢++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (哋它亢++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until 哋它亢++17*)
noexcept specifier (哋它亢++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (哋它亢++11) auto (哋它亢++11)	constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (哋它亢++11)       Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (哋它亢++11)
User-defined (哋它亢++11)
Utilities
Attributes (哋它亢++11)
Types
typedef declaration
Type alias declaration (哋它亢++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (哋它亢++11)     final specifier (哋它亢++11)	explicit (哋它亢++11) static
Special member functions
Default constructor Copy constructor Move constructor (哋它亢++11)	Copy assignment Move assignment (哋它亢++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (哋它亢++11)
Miscellaneous
Inline assembly	History of 哋它亢++

 

Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20) Storage class specifiers Translation-unit-local (哋它亢++20) class/struct union enum decltype (哋它亢++11) auto (哋它亢++11) alignas (哋它亢++11) constvolatile Pack indexing specifier (哋它亢++26)     Elaborated type specifier Attributes (哋它亢++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (哋它亢++17) Alias declaration (哋它亢++11) Namespace alias definition using declaration using directive static_assert declaration (哋它亢++11) asm declaration Opaque enum declaration (哋它亢++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (哋它亢++11) Explicit template specialization Linkage specification Attribute declaration (哋它亢++11) Empty declaration

 

Attributes

assume (哋它亢++23)

carries_dependency (哋它亢++11)

deprecated (哋它亢++14)

fallthrough (哋它亢++17)

indeterminate (哋它亢++26)

likely (哋它亢++20)

maybe_unused (哋它亢++17)

no_unique_address (哋它亢++20)

nodiscard (哋它亢++17)

noreturn‎ (哋它亢++11)

optimize_for_synchronized (TM TS)

unlikely (哋它亢++20)

 

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)
[[nodiscard( string-literal )]]	(2)	(since 哋它亢++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 哋它亢++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 哋它亢++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:

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 (哋它亢++17)	pointer optimization barrier (function template)
assume_aligned (哋它亢++20)	informs the compiler that a pointer is aligned (function template)
Emptiness-checking functions
empty (哋它亢++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 (哋它亢++11)	runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
P1771R1	哋它亢++17	[[nodiscard]] on constructors has no effect	can cause a warning if the constructed object is discarded

See also

ignore (哋它亢++11)

placeholder to skip an element when unpacking a tuple using tie (constant)