convert string to integer in c++

Question

Hello I know it was asked many times but I hadn\'t found answer to my specific question.

I want to convert only string that contains only decimal numbers:

Fo

Answer 1

An other way using c++ style : We check the number of digits to know if the string was valid or not :

#include 
#include 
#include 
#include 

int main(int argc,char* argv[]) {

    std::string a("256");

    std::istringstream buffer(a);
    int number;
    buffer >> number; // OK conversion is done !
    // Let's now check if the string was valid !
    // Quick way to compute number of digits
    size_t num_of_digits = (size_t)floor( log10( abs( number ) ) ) + 1;
    if (num_of_digits!=a.length()) {
        std::cout << "Not a valid string !" << std::endl;
    }
    else {
        std::cout << "Valid conversion to " << number  << std::endl;
    }

}