View Full Version : Check C++ variable type

December 2nd, 2008, 03:47 AM
So I am currently learning C++ and decided to make a program that tests my skills I have learned so far. Now in my code I want to check if the value that the user enters is a double, if it is not a double I will put a if loop and ask them to reenter it. The problem I have is how do I go about checking what type of variable the user enters, ex- if a user enters a char or string, I can output an error message. Here is my code:

//cubes a user entered number
#include <iostream>
using namespace std;

double cube(double n); //function prototype

int main()
cout << "Enter the number you want to cube: "; //ask user to input number
double user;
cin >> user; //user entering the number

cout << "The cube of " << user << " is " << cube(user) << "." << endl; //displaying the cubed number

return 0;

double cube (double n) //function that cubes the number
return n*n*n; // cubing the number and returning it

December 2nd, 2008, 05:50 AM
I won't write any code but

1.234566 % 1 == .234566
1%1 == 0

while (num%1 == 0) {
prompt for a double

this works for floats too, why do you need to make sure it is a double (decimal number?), if you assign a value to a double then it is a double

December 2nd, 2008, 08:01 AM
double user;
cin >> user; //user entering the number

You should check the result of the 'cin >> user' operation and/or the state of the stream after trying to read. If the data in the stream cannot be converted into a stream (it is a character) then the operation will fail (this can be checked either converting the stream to a bool [implicitly in an if suffices] or directly querying the stream:

// option 1
if ( !(cin >> user) ) {

// option 2
cin >> user;
if ( cin.fail() ) {

After checking that the read failed you should clear the error bits in the stream (.clear()) and clear the offending data out of the buffer (if the operation failed then the data is not removed from the stream).

December 2nd, 2008, 11:25 AM
You can't tell if a number entered (typed on a keyboard) is a double or not, the concept doesn't exist. It's only a double, float or int if you make it so in the program.
-1, 1, 1.1, 1.111111111 could all be doubles if you want them to be.
Cheers, Mike

December 2nd, 2008, 04:43 PM
you can always use C++ RTTI ability :

int i;

// ...

typeid(i) == typeid(int) // True

typeid(8) == typeid(int) // True

// ...

use 'typeid' as shown above.

December 2nd, 2008, 11:09 PM
you can always use C++ RTTI ability :

use 'typeid' as shown above.

How does that check what the user has entered"?