class template
<functional>
std::unary_function
template <class Arg, class Result> struct unary_function;
template <class Arg, class Result> struct unary_function; // deprecated
Unary function object base class
Note: This class has been deprecated in C++11.
This is a base class for standard unary function objects.
Generically, function objects are instances of a class with member function operator() defined. This member function allows the object to be used with the same syntax as a regular function call, and therefore its type can be used as template parameter when a generic function type is expected.
In the case of unary function objects, this operator() member function takes a single parameter.
unary_function is just a base class, from which specific unary function objects are derived. It has no operator() member defined (which derived classes are expected to define) - it simply has two public data members that are typedefs of the template parameters. It is defined as:
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template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
};
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Member types
| member type | definition | notes |
|---|
| argument_type | The first template parameter (Arg) | Type of the argument in member operator() |
| result_type | The second template parameter (Result) | Type returned by member operator() |
Example
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// unary_function example
#include <iostream> // std::cout, std::cin
#include <functional> // std::unary_function
struct IsOdd : public std::unary_function<int,bool> {
bool operator() (int number) {return (number%2!=0);}
};
int main () {
IsOdd IsOdd_object;
IsOdd::argument_type input;
IsOdd::result_type result;
std::cout << "Please enter a number: ";
std::cin >> input;
result = IsOdd_object (input);
std::cout << "Number " << input << " is " << (result?"odd":"even") << ".\n";
return 0;
}
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Possible output:
Please enter a number: 2
Number 2 is even.
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