---
description: "Learn more about: equal_to Struct"
title: "equal_to Struct"
ms.date: "11/04/2016"
f1_keywords: ["functional/std::equal_to"]
helpviewer_keywords: ["equal_to function", "equal_to struct"]
ms.assetid: 8e4f2b50-b2db-48e3-b4cc-6cc03362c2a6
---
# equal_to Struct

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

## Syntax

```cpp
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

```cpp
// 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;
}
```

```Output
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 )
```