equal_to Struct

Article
01/02/2018

The latest version of this topic can be found at equal_to Struct.

A binary predicate that performs the equality operation ( operator==) on its arguments.

Syntax

template <class Type = void>  
struct equal_to : public binary_function<Type, Type, bool>   
 {  
    bool operator()(
    const Type& Left,   
    const Type& Right) const;

 
 };  
 
// specialized transparent functor for operator== 
template <>  
struct equal_to<void>  
 {  
    template <class T, class U>  
auto operator()(T&& Left, U&& Right) const 
 ->  
decltype(std::forward<T>(Left)  
 == std::forward<U>(Right));

 };

Parameters

Type, T, U
Any type that supports an operator== that takes operands of the specified or inferred types.

Left
The left operand of the equality operation. The unspecialized template takes an lvalue reference argument of type Type. The specialized template does perfect forwarding of lvalue and rvalue reference arguments of inferred type T.

Right
The right operand of the equality operation. The unspecialized template takes an lvalue reference argument of type Type. The specialized template does perfect forwarding of lvalue and rvalue reference arguments of inferred type U.

Return Value

The result of Left``==``Right. The specialized template does perfect forwarding of the result, which has the type that's returned by operator==.

Remarks

The objects of type Type must be equality-comparable. This requires that the operator== defined on the set of objects satisfies the mathematical properties of an equivalence relation. All of the built-in numeric and pointer types satisfy this requirement.

Example

// functional_equal_to.cpp  
// compile with: /EHsc  
#include <vector>  
#include <functional>  
#include <algorithm>  
#include <iostream>  
  
using namespace std;  
  
int main( )  
{  
   vector <double> v1, v2, v3 ( 6 );  
   vector <double>::iterator Iter1, Iter2, Iter3;  
  
   int i;  
   for ( i = 0 ; i <= 5 ; i+=2 )  
   {  
      v1.push_back( 2.0 *i );  
      v1.push_back( 2.0 * i + 1.0 );  
   }  
  
   int j;  
   for ( j = 0 ; j <= 5 ; j+=2 )  
   {  
      v2.push_back( - 2.0 * j );  
      v2.push_back( 2.0 * j + 1.0 );  
   }  
  
   cout << "The vector v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   cout << "The vector v2 = ( " ;  
   for ( Iter2 = v2.begin( ) ; Iter2 != v2.end( ) ; Iter2++ )  
      cout << *Iter2 << " ";  
   cout << ")" << endl;  
  
   // Testing for the element-wise equality between v1 & v2  
   transform ( v1.begin( ),  v1.end( ), v2.begin( ), v3.begin ( ),   
      equal_to<double>( ) );  
  
   cout << "The result of the element-wise equal_to comparison\n"  
      << "between v1 & v2 is: ( " ;  
   for ( Iter3 = v3.begin( ) ; Iter3 != v3.end( ) ; Iter3++ )  
      cout << *Iter3 << " ";  
   cout << ")" << endl;  
}

The vector v1 = ( 0 1 4 5 8 9 )  
The vector v2 = ( -0 1 -4 5 -8 9 )  
The result of the element-wise equal_to comparison  
between v1 & v2 is: ( 1 1 0 1 0 1 )

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