C++ ONP Calculator

#include <stdexcept>
#include <limits>
#include <iostream>
#include <string>
#include <vector>
/// <summary>
/// Waits for any key being clicked
/// </summary>
void WaitForAnyKey() {
#if defined _WIN32
    system("pause");
#elif defined (__LINUX__) || defined(__gnu_linux__) || defined(__linux__)
    system("read");
#elif defined (__APPLE__)
    system("read");
#endif
}
/// <summary>
/// Clear Console
/// </summary>
void Clear()
{
#if defined _WIN32
    system("cls");
#elif defined (__LINUX__) || defined(__gnu_linux__) || defined(__linux__)
    system("clear");
#elif defined (__APPLE__)
    system("clear");
#endif
}
/// <summary>
/// Check If Char contains digit (by ASCII check)
/// </summary>
/// <param name="c">syntax</param>
/// <returns></returns>
bool IsDigit(char c) {
    return ('0' <= c) && (c <= '9');
}
/// <summary>
/// Calculate the value of ONP expression
/// </summary>
/// <param name="expression">Expression in string</param>
/// <returns></returns>
double ONP(std::string expression) {
    if (expression.size() > 0) {
        std::vector<double> _tower;
        for (char c : expression) {
            if (c != ' ') {
                if (IsDigit(c))
                    _tower.push_back(c-'0');
                else {
                    switch (c) {
                    case '+':
                        if (_tower.size() > 1) {
                            _tower.push_back(_tower[_tower.size() - 2] + _tower[_tower.size() - 1]);
                            // element added so indexes changes
                            _tower.erase(_tower.begin() + _tower.size() - 3);
                            _tower.erase(_tower.begin() + _tower.size() - 2);
                        }
                        else
                            throw std::invalid_argument("Expression is invalid!");
                        break;
                    case '-':
                        if (_tower.size() > 1) {
                            _tower.push_back(_tower[_tower.size() - 2] - _tower[_tower.size() - 1]);
                            // element added so indexes changes
                            _tower.erase(_tower.begin() + _tower.size() - 3);
                            _tower.erase(_tower.begin() + _tower.size() - 2);
                        }
                        else
                            throw std::invalid_argument("Expression is invalid!");
                        break;
                    case '*':
                        if (_tower.size() > 1) {
                            _tower.push_back(_tower[_tower.size() - 2] * _tower[_tower.size() - 1]);
                            // element added so indexes changes
                            _tower.erase(_tower.begin() + _tower.size() - 3);
                            _tower.erase(_tower.begin() + _tower.size() - 2);
                        }
                        else
                            throw std::invalid_argument("Expression is invalid!");
                        break;
                    case '/':
                        if (_tower.size() > 1) {
                            _tower.push_back(_tower[_tower.size() - 2] / _tower[_tower.size() - 1]);
                            // element added so indexes changes
                            _tower.erase(_tower.begin() + _tower.size() - 3);
                            _tower.erase(_tower.begin() + _tower.size() - 2);
                        }
                        else
                            throw std::invalid_argument("Expression is invalid!");
                        break;
                    default:
                        throw std::invalid_argument("Expression is invalid!");
                    }
                }
            }
        }
        if (_tower.size() == 1)
            return _tower[0];
    }
    throw std::invalid_argument("Expression is invalid!");
}
int main()
{
    while (true) {
        std::cout << "Enter ONP expression:" << std::endl;
        std::string expression;
        std::getline(std::cin,expression);
        try {
            std::cout << "Expression's value equals to " << ONP(expression) << "." << std::endl;
        }
        catch (std::invalid_argument const& ex) {
            std::cout << "Exception occured: " << ex.what() << std::endl;
        }
        WaitForAnyKey();
        Clear();
    }
}