Question

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 } 

Edit: I would have to say I just started and don’t have the slightest of clue about your code, but I will check out your link. By the way, I haven’t learned how to work with templates yet,I am learning about dealing with data, only Chapter 3 in my C++ Primer Plus 5th edition.

Answer 1

There is no suitable way to check if a string really contains a double within the standard library. You probably want to use Boost. The following solution is inspired by recipe 3.3 in C++ Cookbook:

#include <iostream> #include <boost/lexical_cast.hpp> using namespace std; using namespace boost;  double cube(double n);  int main() {     while(true)     {         cout << 'Enter the number you want to cube: ';         string user;         cin >> user;          try         {             // The following instruction tries to parse a double from the 'user' string.             // If the parsing fails, it raises an exception of type bad_lexical_cast.             // If an exception is raised within a try{ } block, the execution proceeds             // with one of the following catch() blocks             double d = lexical_cast <double> (user);                 cout << 'The cube of ' << d << ' is ' << cube(d) << '.' << endl;             break;         }         catch(bad_lexical_cast &e)         {             // This code is executed if the lexical_cast raised an exception; We             // put an error message and continue with the loop             cout << 'The inserted string was not a valid double!' << endl;         }     }     return 0; }  double cube (double n) {     return n*n*n; }