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

Answer 1

if(x) is equivalent to if(bool(x))

in this case bool(x) calls std::istream::operator bool(x)

this will return:

true if none of failbit or badbit is set.

false otherwise.

Answer 2

What is inside if condition will be evaluated to bool.

if(cin >> X) means that if condition is true, something was read to X; if condition is false, something else happened (e.g. stream ended) and X is not changed.

E.g. to read until the end of stream, you can use while(cin >> X).

Answer 3

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.

Answer 4

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.