How does cin evaluate to true when inside an if statement?

Question

I thought that:

if (true) 
{execute this statement}

So how does if (std::cin >> X) execute as true when there is nothing "true" about it? I could understand if it was if ( x <= y) or if ( y [operator] x ), but what kind of logic is "istream = true?".

Answer 1

The answer depends on the version of the standard C++ library:

• Prior to C++11 the conversion inside if relied on converting the stream to void* using operator void*
• Starting with C++11 the conversion relies on operator bool of std::istream

Note that std::cin >> X is not only a statement, but also an expression. It returns std::cin. This behavior is required for "chained" input, e.g. std::cin >> X >> Y >> Z. The same behavior comes in handy when you place input inside an if: the resultant stream gets passed to operator bool or operator void*, so a boolean value gets fed to the conditional.

Answer 2

std::cin is of type std::basic_istream which inherits from std::basic_ios, which has an operator : std::basic_ios::operator bool which is called when used in if statement.