[Solved] How do I check in this code if the number is in integer?

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You can use boost lexical-cast which is exactly for this purpose. It will throw an exception the conversion fails. Boost is well tested and you can safly use it to do the conversion for you.

This could look like this: 

#include <boost/lexical_cast.hpp>
#include <iostream>


int cube(int x)
{
    return x*x*x;
}

int main()
{
    std::string x;
    std::cout << " Enter an integrer : ";
    std::cin >> x;
    try
    {
        int y = boost::lexical_cast<int>(x);
        int cube_x = cube(y);
        std::cout << "Cube(" << x << ")=" << cube_x << std::endl;
    }
    catch (const boost::bad_lexical_cast &e)
    {
        std::cerr << e.what() << '\n';
    }
    return 0;
}

By the way, if your program shall only handle integers, you should also use type int and not float to handle the numbers.

solved How do I check in this code if the number is in integer?