isdigit() isn't working in a simple program?

Question

#include<iostream.h>
#include<conio.h>
#include<ctype.h>
void main()
{
    clrscr();
    int a;
    cout<<"Enter a digit";
    cin>>a;
    if(isdigit(a))
    {
         cout<<"You have entered a digit";
    }
    else
    {
         cout<<"Not a digit";
    }
    getch();
}

The code shows no errors, but every time I enter a digit it displays "Not a digit".

Answer 1

int a;
std::cin >> a;

This code (or your somewhat antiquated variant) reads text from the console and converts that text into an integer value. So if you type 0 at the console, the value of a will be 0, not '0'. isdigit tells you whether the character value that you pass to it represents a digit, and 0 does not, so the result is almost certainly correct. If you instead read the value into a variable of type char you'll get the behavior that you expect.

char a;
std::cin >> a;