c++ - 使用 openmode ios_base::out 的 istringstream 有什么意义

Question

istringstream 的目的是什么？构造函数参数 openmode ？

特别是指定 ios_base::out 有什么意义吗？因为我认为这个对象从不支持流插入 <<运算符(operator)？

Answer 1

具有ios_base::in 访问模式的流不支持任何输出操作。如果未指定 ios_base::out，则更改序列的方法将失败。

来自 cppreference

Open mode: Access given by the internal stringbuf object to its internal sequence of characters.

ios_base::out - output - The sequence supports output operations.

ios_base::in is always set for istringstream objects (even if explicitly not set in argument which). Note that even though istringstream is an input stream, its internal stringbuf object may be set to also support output operations. This influences certain operations, such as putback, that in istringstream may alter the contents of the sequence.

参见 putback example :

        std::istringstream s1("Hello, world", std::ios_base::out); //  stream supporting output operations
        s1.get();
        if (s1.putback('Y')) // modifies the buffer
            std::cout << s1.rdbuf() << '\n';
        else
            std::cout << "putback failed\n";

        std::istringstream s2("Hello, world"); // input-only stream
        s2.get();
        if (s2.putback('Y')) // cannot modify input-only buffer
            std::cout << s2.rdbuf() << '\n';
        else
            std::cout << "putback failed\n";

        s2.clear();
        if (s2.putback('H')) // non-modifying putback is OK
            std::cout << s2.rdbuf() << '\n';
        else
            std::cout << "putback failed\n";