C++: how is istream is converted to bool inside a conditional expression [duplicate]

问题

This question already has answers here:

Why istream object can be used as a bool expression? (2 answers)

Closed 6 years ago.

The istream operator>> is used to read the data and the function returns reference to istream.

For example,

istream& operator>> (bool& val);

But how does the istream is converted into a bool when it is used inside the conditional statement.

For example,

ifstream ifs(.....);  // open the file
istream &is = (istream&)ifs;

char c;

if(is >> c)   // how the istream is been evaluated into as bool
{
    // character read
}

Can anyone explain how it is being converted into a bool inside a conditional expression?

回答1:

From cppreference:

   explicit std::basic_ios::operator bool() const;

Returns true if the stream has no errors occurred and is ready of I/O operations. Specifically, returns !fail().

So since an if statement is a boolean context, it will invoke std::istream's member function operator.

回答2:

operator >> return a reference to istream (istream&).

and so you're actually writing if (istream) which is in turn calling the operator bool..

in conditionals 0 is false anything else is true -> istream has an opeartor bool that checks if the stream is ok so it will return !fail()...hence true.

来源：https://stackoverflow.com/questions/16777451/c-how-is-istream-is-converted-to-bool-inside-a-conditional-expression