std::string ex = std::string("3+4");
int pos = ex.find_first_of("+-*/"); // 1

while (pos != std::string::npos) {
    std::string prenxn = ex.substr(pos + 1); // 4

    int pnf = prenxn.find_first_of("+-*/"); // std::string::npos

    if (pnf != std::string::npos)
        prenxn = prenxn.substr(0, pnf);

    int nxn = std::stoi(prenxn);

    switch (ex[pos]) {
        case '+':
            res += nxn;

        case '-':
            res -= nxn;

        case '*':
            res *= nxn;

        case '/':
            res /= nxn;

        default:
            char* expos = &ex[pos];
            throw std::runtime_error(expos);
    }

    pos = ex.find_first_of("+-*/", pos + 1);
}