How to implement throw statement to do a integer value - c++

Question

I have a question that might be very simple to many of you however, I have not found an answer to my question.
I have the program below that is working properly. This code turns a number into a floating number and integer.
Let's say you entered 5.4, the program will give you 5.4 for double and 5 for integer.
Now I need to add a throw catch statement to my program in case the user enters a text instead of a number ("If the conversion fails, throw an exception and allow the user to re-enter the value.").
This is the pseudocode of what I need to do.
try {
if(num ==character)
throw;
cout << "\n The number entered " << num << "invalid, please enter again";
}
catch
{
}
and I implemented something like this, however it did not work. I set the 'a' variable character thinking the user has to enter a text in order to get that message. However it did not work and gave some errors.
try
{
char a;
if (num == a)
throw num;
}
catch(int e)
{
cout << "A number of " << a << " is invalid." << endl;
cout << "Please re-enter a number: ";
cin << num
}
I am very new with this "try,throw,catch" terms. I would be happy if you help me through this, thanks.
#include <C:\\CSIS1600\MyCppUtils.cpp>
#include <iostream>
#include <string>
using namespace myNameSpace;
int main()
{
runner("is running");
cout << "Enter a number : ";
string num;
getline(cin, num);
cout<< "double " << getValidDouble(num) << endl;
cout<< "integer " << getValidInt(num) << endl;
system("pause");
return 0;
}
#include<iostream>
#include<string>
using namespace std;
namespace myNameSpace
{
string num;
void runner(string str)
{
cout <<"runner-3() is running.."<<endl;
}
int getValidInt(string n)
{
int valueint;
valueint=atoi(n.c_str());
return valueint;
}
double getValidDouble(string n )
{
double valuedouble;
valuedouble = atof(n.c_str());
return valuedouble;
}
}

Answer 1

You can use Boost to do a lexical cast. If you have valid input (e.g. 2.54), no exception will be thrown, but with invalid input (e.g. 2???54) the bad_lexical cast is thrown:
#include <boost/lexical_cast.hpp>
try
{
double x1 = boost::lexical_cast<double> ("2.54");
double x2 = boost::lexical_cast<double> ("2???54");
cout << x1 << x2 << endl;
}
catch(boost::bad_lexical_cast& e)
{
cout << "Exception caught - " << e.what() << endl;
}