Modernes C++ programmieren

// https://godbolt.org/z/5oe38obPr 
#include <string>
#include <vector>
#include <iostream>       // cout

void rechner(std::ostream& out, std::string input) {
    std::vector<int> stapel {};
    for(char c : input) {
        if(c>='0' && c<='9') {
            stapel.push_back( c-'0' ); // Zahlenwert des Zeichens
            continue;     // nächste Schleifeniteration
        }
        int top = 0;
        int second = 0;
        switch(c) {       // Bedingung auf zeichen
        case '+':
            top = stapel.back(); stapel.pop_back();
            second = stapel.back(); stapel.pop_back();
            stapel.push_back(second + top);
            break;
        case '-':
            top = stapel.back(); stapel.pop_back();
            second = stapel.back(); stapel.pop_back();
            stapel.push_back(second - top);
            break;
        case '*':
            top = stapel.back(); stapel.pop_back();
            second = stapel.back(); stapel.pop_back();
            stapel.push_back(second * top);
            break;
        case '=':
            for(int elem : stapel) { out << elem; }
            out << "\n";
            break;
        case ' ':
            break;
        default:
            out << "\n'" << c << "' verstehe ich nicht.\n";
        } /* switch */
    } /* for c */
}
int main(int argc, const char* argv[]) {
    if(argc > 1) {
        rechner(std::cout, argv[1]);
    } else {
        // 3 + 4 * 5 + 6 mit Punkt- vor Strichrechnung ergibt 29
        rechner(std::cout, "345*+6+=");
    }
}