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

Question

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?

`ifstream` is derived from `istream`? – John Jun 21 '22 at 01:24 — John, Jun 21 '22 at 01:24

score 10 · Answer 1 · edited Jun 20 '20 at 09:12

10

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.

edited Jun 20 '20 at 09:12

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answered May 27 '13 at 16:55

David G

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`ifstream` is derived from `istream`? – John Jun 21 '22 at 01:24
1

@John Yes it is – David G Jun 21 '22 at 01:29

score 0 · Answer 2 · answered May 27 '13 at 16:57

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.

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

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