Question

I recently started learning DirectX/Windows, and the book I’m learning from had the code

d3d = Direct3DCreate9(D3D_SDK_VERSION);
if(d3d == NULL)
    //catch error &c.

My question is: What would cause an error in this line, that is different than what would cause an error in another line (say, for example, int num = 42)?

Answer 1

d3d = Direct3DCreate9(D3D_SDK_VERSION);
if (d3d == NULL)

This is an error or not according to the meaning you give to the return value of Direct3DCreate9, i.e. depending on the specification of the function. I’ve written many pointer-returning functions for which NULL as a return value was not an erroneous situation.

So, do not equate “a function returning NULL” to “an error”. An unambiguous error is a crash (technically, undefined behaviour) in your code, like if d3d is indeed NULL and later you dereference it.

int num = 42;

Here you are declaring an int variable called num and initializing it with a value of 42. What kind of error can you think of? Obviously, num will never “be NULL”, if that bothers you. 42 may be a correct value or an error, depending on the context.