How would std::ostringstream convert to bool?

问题:

I stumbled across this code.

    std::ostringstream str;
    /// (some usage)
    assert( ! str );

What does ostringstream signify when used in a bool context?

Is this possibly an incorrect usage that happens to compile and run?

回答1:

It tells you if the stream is currently valid. This is something that all streams can do. A file stream, for example, can be invalid if the file was not opened properly.

As a side note, this functionality (testing a stream as a bool) is achieved by overloading explicit operator bool in C++11 and later and by overloading the void* cast operator in versions before C++11.

Here is a link containing some examples of why a stream might fail. This isn't specific to string streams, but it does apply to them.

Edit: changed bool to void* after Martin York pointed out my mistake.

回答2:

For reference: ostringstream::operator void*() and ostringstream::operator!().

回答3:

The expression is valid and evaluates the state of the stream. This feature is more commonly used on input streams:

istringstream is;
is.str( "foo" );
int x;
is >> x;

if ( ! is ) {
   cerr << "Conversion failed";
}

I'm not sure how any of the standard streaming functions could cause an ostringstream to go bad, but you could certainly write one yourself.