Namespaces
Variants
Views
Actions

std::unary_function

From cppreference.com
< cpp‎ | utility‎ | functional
 
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
(C++26)
Three-way comparison
(C++20)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
General utilities
Date and time
Function objects
Formatting library (C++20)
(C++11)
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)   
(C++20)
Swap and type operations
(C++14)
(C++11)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)
Elementary string conversions
(C++17)
(C++17)

 
Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

Old binders and adaptors
unary_function
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
Defined in header <functional>
template< typename ArgumentType, typename ResultType >
struct unary_function;
(deprecated in C++11)
(removed in C++17)

std::unary_function is a base class for creating function objects with one argument.

std::unary_function does not define operator(); it is expected that derived classes will define this. std::unary_function provides only two types - argument_type and result_type - defined by the template parameters.

Some standard library function object adaptors, such as std::not1, require the function objects they adapt to have certain types defined; std::not1 requires the function object being adapted to have a type named argument_type. Deriving function objects that take one argument from std::unary_function is an easy way to make them compatible with those adaptors.

std::unary_function is deprecated in C++11.

[edit] Member types

Type Definition
argument_type ArgumentType
result_type ResultType

[edit] Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>
 
struct less_than_7 : std::unary_function<int, bool>
{
    bool operator()(int i) const { return i < 7; }
};
 
int main()
{
    std::vector<int> v(10, 7);
    v[0] = v[1] = v[2] = 6;
 
    std::cout << std::count_if(v.begin(), v.end(), std::not1(less_than_7()));
 
    // C++11 solution:
    // Cast to std::function<bool (int)> somehow - even with a lambda
    // std::cout << std::count_if(v.begin(), v.end(),
    //     std::not1(std::function<bool (int)>([](int i) { return i < 7; })));
}

Output:

7

[edit] See also

(C++11)
wraps callable object of any copy constructible type with specified function call signature
(class template) [edit]
wraps callable object of any type with specified function call signature
(class template) [edit]
(deprecated in C++11)(removed in C++17)
creates an adaptor-compatible function object wrapper from a pointer to function
(function template) [edit]
(deprecated in C++11)(removed in C++17)
adaptor-compatible wrapper for a pointer to unary function
(class template) [edit]
(deprecated in C++11)(removed in C++17)
adaptor-compatible binary function base class
(class template) [edit]