僅接受用戶的數字輸入

template <typename T>
T readFromLine(std::istream& in)
{
    std::string line;
    while (std::getline(in, line)) { // read whole line until it is possible
        std::istringstream line_stream { line }; // initialize string stream to parse line read
        T x;

        // try read `x` then discard white spaces then check if end of lien was reached
        if (line_stream >> x >> std::ws && line_stream.eof()) {
            // stop since reading was successful and line do not contain anything extra
            return x;
        }
        // continue if reading `x` was unsuccessful or when line contains something extra
    }
    // while loop ended since end of file was reached or error on input stream
    throw std::invalid_argument { "stream to short" };
}

https://godbolt.org/z/75efEP9Y8