Check if the input is a number or string in C++

Question

I wrote the following code to check whether the input(answer3) is a number or string, if it is not a number it should return "Enter Numbers Only" but it returns the same even for numbers. Please suggest me a solution.

#include <iostream>
#include <string>
#include <typeinfo>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

using namespace std; 
int main ()
{

string ques1= "Client's Name :";
string ques2 = "Client's Address :";
string ques3 = "Mobile Number :";

char answer1 [80];
string answer2;
int answer3;

     cout<<ques1<<endl;    
     cin>>answer1;      

     cout<<ques2<<endl;    
     cin>>answer2; 

     cout<<ques3<<endl;
     cin>>answer3;

       if (isdigit(answer3))
       {
              cout<<"Correct"<<endl;     

              }

        else
        {
          cout<<"Enter Numbers Only"<<endl;  

            }

 system("pause>null");
 return 0;  

}

Answer 1

You can use regex to do this:

#include <regex>

bool isNumber(std::string x){
    std::regex e ("^-?\\d+");
    if (std::regex_match (x,e)) return true;
    else return false;}

If you want to make isNumber() a generic function which can take any type of input:

#include <regex>
#include <sstream>

template<typename T>
bool isNumber(T x){
    std::string s;
    std::regex e ("^-?\\d+");
    std::stringstream ss; 
    ss << x;
    ss >>s;
    if (std::regex_match (s,e)) return true;
    else return false;}

The above isNumber() function checks for integer only, double or float value with precision (which contains dot .) will not return true. If you want precision too, then change the regex line to:

std::regex e ("^-?\\d*\\.?\\d+");

If you want a more efficient solution, see this one.