Простой калькулятор рациональных чисел

#include <iostream>
#include <conio.h>
#include <string>

using namespace std;

enum choice {
    ESC      = 27,
    INCREASE = 42,
    COMBINE  = 43,
    SUBTRACT = 45,
    DIVIDE   = 47
};

struct rational {
    int numerator;
    int denominator;
};
typedef rational RATIONAL;

void combine(const RATIONAL, const RATIONAL);
void subtract(const RATIONAL, const RATIONAL);
void increase(const RATIONAL, const RATIONAL);
void divide(const RATIONAL, const RATIONAL);
int  nod(int, int);
void reduce(RATIONAL &);
void show(const RATIONAL, const RATIONAL, const RATIONAL, const string);
int  validint(const RATIONAL, const RATIONAL);
char validchar();
void err();
void zero();
void ui();
void ru();

int main() {
    ru();
    ui();
    return 0;
}

void reduce(RATIONAL & _rat) {
    int n = nod(_rat.numerator, _rat.denominator);

    if (n) {
        _rat.numerator /= n;
        _rat.denominator /= n;
    }
}

int nod(int _a, int _b) {
    int a = abs(_a);
    int b = abs(_b);

    if (!a && !b) a = 0;
    else while (b) b ^= a ^= b ^= a %= b;

    return a;
}

void combine(const RATIONAL _first, const RATIONAL _second) {
    RATIONAL result;
    result.numerator   = _first.numerator * _second.denominator + _second.numerator * _first.denominator;
    result.denominator = _second.denominator * _first.denominator;
    reduce(result);
    if (result.denominator) show(_first, _second, result, " + ");
    else zero();
}

void subtract(const RATIONAL _first, const RATIONAL _second) {
    RATIONAL result;
    result.numerator   = _first.numerator * _second.denominator - _second.numerator * _first.denominator;
    result.denominator = _second.denominator * _first.denominator;
    reduce(result);
    if (result.denominator) show(_first, _second, result, " - ");
    else zero();
}

void increase(const RATIONAL _first, const RATIONAL _second) {
    RATIONAL result;
    result.numerator   = _first.numerator * _second.numerator;
    result.denominator = _first.denominator * _second.denominator;
    reduce(result);
    if (result.denominator) show(_first, _second, result, " * ");
    else zero();
}

void divide(const RATIONAL _first, const RATIONAL _second) {
    RATIONAL result;
    result.numerator   = _first.numerator * _second.denominator;
    result.denominator = _first.denominator * _second.numerator;
    reduce(result);
    if (result.denominator) show(_first, _second, result, " / ");
    else zero();
}
void zero() { wcout << L"Ошибка! Попытка деления на ноль\n\a"; }

void show(const RATIONAL _first, const RATIONAL _second, const RATIONAL _result, const string _op) {
    cout << _first.numerator  << '/' << _first.denominator  << _op
         << _second.numerator << '/' << _second.denominator << " = ";
    if ((_op == " * " && (!_first.numerator || !_second.numerator)) || !_first.numerator && !_second.numerator) cout << 0;
    else cout << _result.numerator << '/' << _result.denominator;
    cout << endl;
}

char validchar() {
    do {
        fflush(stdin);
        char op = cin.get();

        if (op == INCREASE || op == COMBINE || op == SUBTRACT || op == DIVIDE) return op;
        else err();
    } while (true);
}

int validint() {
    int num;

    do {
        cin >> num;
        if (cin.good()) return num;
        else err();
    } while (true);
}

void err() {
    cin.clear();
    cin.ignore(80,'\n');
    wcout << L"\a Ошибка! Повторите попытку:       ";
}

void ui() {
    RATIONAL first, second;

    do {
        wcout << L"\nВведите числитель первой дроби:  ";
        first.numerator = validint();
        wcout << L"Введите знаменатель первой дроби: ";
        first.denominator = validint();
        wcout << L"Введите арифметическую операцию:  ";
        char choice = validchar();
        wcout << L"Введите числитель второй дроби:  ";
        second.numerator = validint();
        wcout << L"Введите знаменатель второй дроби: ";
        second.denominator = validint();

        switch (choice) {
            case COMBINE:  combine(first, second);  break;
            case SUBTRACT: subtract(first, second); break;
            case INCREASE: increase(first, second); break;
            case DIVIDE:   divide(first, second);   break;
        }

        wcout << L"\nДля выхода нажмите клавишу ESC\nДля продолжения любую другую клавишу";
        fflush(stdin);
        if (ESC == _getch()) break;
        else cout << endl;
    } while (true);
}

void ru() {
    wcout.imbue(locale(".866"));
    wcin.imbue(locale(".866"));
}