std::getline() 如何等同于 bool？

Question

我是 std::getline(...) 处女，在 cppreference.com 上查阅了文档和示例，我对这样的示例代码感到困惑：

#include <sstream>
#include <string>

int main(int argc, char* argv[])
{
    std::string line;
    std::ifstream infile("sample.txt");

    while (std::getline(infile, line))
    {
        // Do stuff
    }

    return 0;
}

...特别是 while 语句：while (std::getline(infile, line)).

提到的文档说 std::getline(std::basic_istream<CharT,Traits>& input, ...) 的 return 值是 input，即对第一个参数的引用。

那么，getline的return值如何作为while循环的条件，需要类型为bool？

std::ifstream 是否实现了 operator bool()？

Answer 1

Does std::ifstream implement an operator bool()?

It does:

Checks whether the stream has no errors. <...> Returns true if the stream has no errors and is ready for I/O operations. Specifically, returns !fail().

This operator makes it possible to use streams and functions that return references to streams as loop conditions, resulting in the idiomatic C++ input loops such as while(stream >> value) {...} or while(getline(stream, string)){...}

std::getline() 如何等同于 bool？

How is std::getline() equated with bool?

c++

stl

getline

conversion-operator

operator-keyword