is_const Template in C++

The std::is_const template of C++ STL is used to check whether the type is a const-qualified or not. It returns a boolean value showing the same.

Syntax:

template  < class T >struct is_const;

Template Parameter: This template contains single parameter T (Trait class) to check whether T is a const-qualified type.

Return Value: This template returns a boolean value as shown below:

• True: if the type is a const-qualified.
• False: if the type is a non-const-qualified.

Below programs illustrate the is_const template in C++ STL:

Program 1:

// C++ program to illustrate
// is_const

#include <iostream>
#include <type_traits>
using namespace std;

// main program
int main()
{
    cout << boolalpha;
    cout << "is_const:"
         << endl;
    cout << "int:"
         << is_const<int>::value
         << '\n';
    cout << "const int:"
         << is_const<const int>::value
         << '\n';
    cout << "const int&:"
         << is_const<typename remove_reference<const int&>::type>::value
         << '\n';
    return 0;
}

Output:

is_const:
int:false
const int:true
const int&:true

Program 2:

// C++ program to illustrate
// is_const

#include <iostream>
#include <type_traits>
using namespace std;

// main program
int main()
{
    cout << boolalpha;
    cout << "is_const:"
         << endl;
    cout << "const int*:"
         << is_const<const int*>::value
         << '\n';
    cout << "int* const:"
         << is_const<int* const>::value
         << '\n';
    cout << "const int&:"
         << is_const<const int&>::value
         << '\n';

    return 0;
}

Output:

is_const:
const int*:false
int* const:true
const int&:false

Program 3:

// C++ program to illustrate
// is_const

#include <iostream>
#include <type_traits>
using namespace std;

// main program
int main()
{
    cout << boolalpha;
    cout << "is_const:"
         << endl;
    cout << "float:"
         << is_const<float>::value
         << '\n';
    cout << "const float:"
         << is_const<const float>::value
         << '\n';
    cout << "char const:"
         << is_const<char const>::value
         << '\n';
    cout << "double:"
         << is_const<double>::value
         << '\n';

    return 0;
}

Output:

is_const:
float:false
const float:true
char const:true
double:false