std::unary_function

C++

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Concepts library (C++20)

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Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
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pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
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swap
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Function objects

Function wrappers function (C++11) mem_fn (C++11) bad_function_call (C++11)	Bind bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
Function invocation invoke (C++17) Identity function object identity (C++20)	Reference wrappers reference_wrapper (C++11) refcref (C++11)(C++11)
Operator wrappers
plus minus negate multiplies divides modulus logical_and logical_or logical_not	equal_to not_equal_to greater less greater_equal less_equal bit_and bit_or bit_xor bit_not (C++14)
Negators
not_fn (C++17)
Searchers
default_searcher (C++17)
boyer_moore_searcher (C++17)
boyer_moore_horspool_searcher (C++17)
Old binders and adaptors
unary_function (until C++17) binary_function (until C++17) ptr_fun (until C++17) pointer_to_unary_function (until C++17) pointer_to_binary_function (until C++17) mem_fun (until C++17) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17)(until C++17)(until C++17)(until C++17) not1 (until C++20) not2 (until C++20)	binder1stbinder2nd (until C++17)(until C++17) bind1stbind2nd (until C++17)(until C++17) mem_fun_ref (until C++17) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17)(until C++17)(until C++17)(until C++17) unary_negate (until C++20) binary_negate (until C++20)

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

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

unary_function does not define operator(); it is expected that derived classes will define this. 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 unary_function is an easy way to make them compatible with those adaptors.

unary_function is deprecated in C++11.

Member types

Type	Definition
argument_type	ArgumentType
result_type	ResultType

Example

Run this code

#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;
    for (int i = 0; i < 10; ++i) v.push_back(i);

    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:

3

See also

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