How do I more properly integrate an exception into my code, and future code? (C++) -

i've begun programming c++ competitions in school, state, etc. i've not had lot of practice , i'm still fresh c++, , i've run issue programs if character entered instead of numerical value, loop run continuously without asking input. program wrote basic chemical conversion bars:

#include "stdafx.h" using namespace std; class exception : public exception { public:     exception(string m = "exception!") : msg(m){}     ~exception() throw() {}     const char* what() const throw() { return msg.c_str(); }  private:     string msg; }; int _tmain(int argc, _tchar* argv[]) {   // 'n' stands numerator ,  'd' stands denominator     int choice;     float n1;     float n2;     float n3;     float n4;     float d1;     float d2;     float d3; float answer;     while (true)         {         cout << "select how many conversion bars have: " << endl;         cin >> choice;             if (choice == 1)                 {cout << "what starting number?" << endl;                 cin >> n1;                 cout << "what first bar (enter numerator denominator): " << endl;                 cin >> n2;                 cin >> d1;                 answer = n1 * n2 / d1;                 cout << "answer: " << answer << endl << endl;}             else if (choice == 2)             {                 cout << "what starting number?" << endl;                 cin >> n1;                 cout << "what first bar (enter numerator denominator): " << endl;                 cin >> n2;                 cin >> d1;                 cout << "what second bar (enter numerator denominator): " << endl;                 cin >> n3;                 cin >> d2;                 answer = (n1 * n2 * n3) / (d1 * d2);                 cout << "answer: " << answer << endl << endl;             }             else if (choice == 3)             {                 cout << "what starting number?" << endl;                 cin >> n1;                 cout << "what first bar (enter numerator denominator): " << endl;                 cin >> n2;                 cin >> d1;                 cout << "what second bar (enter numerator denominator): " << endl;                 cin >> n3;                 cin >> d2;                 cout << "what third bar (enter numerator denominator): " << endl;                 cin >> n4;                 cin >> d3;                   answer = (n1 * n2 * n3 * n4) / (d1 * d2 * d3);                 cout << "answer: " << answer << endl << endl;             }             else if ((choice /= 1) || (choice /= 2) || (choice /= 3))                 {cout << "that not valid option." << endl << endl;}             try{ throw exception();}                 catch (exception& e)                                 {                 cout << e.what() << endl;                 break;             }         } return 0; } 

and headers:

#pragma once  #include "targetver.h" #include <stdio.h> #include <tchar.h> #include <iostream> #include <cstdint> #include <cstdbool> #include <string> 

so how better integrate exceptions code , future code?

three basic rules exceptions (of course more personal flavor maybe find useful):

1. do not use exceptions control flow - use control flow statements (if, while)

   //good: while(isvalidinput) {     // check validness      // e.g. if (atoi(input) != 1) ... } 

2. do not hide caught exceptions own exception types (you know wrong - hide in upper layers)

   //bad: ... catch(std::ios_base::failure& ex) {     throw mydataformatexception("format error"); } 

3. use exceptions low level possible (you wasting lots of cpu power , can risk not realizing new errors in program because caught in main block)

   //bad: int main() { try { /* */ } catch(...) {} }; // good: try {     std::ifstream inputfilestream("~/file.txt"); } catch(std::ios_base::failure&) { ... }