floating point - Is if(double) valid C++?

Question

I just ran into this line of code:

if( lineDirection.length2() ){...}

where length2 returns a double. It kind of puzzles me that 0.0 is equivalent to 0, NULL, and/or false.

Is this part of the C++ standard or is it undefined behaviour?

Answer 1

It is a very much Standard Behavior (Boolean Conversion)

$4.12/1 - "An rvalue of arithmetic, enumeration, pointer, or pointer to member type can be converted to an rvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true."