What is the meaning of NULL != value in C++? [duplicate]

Answer 1

No, there is no difference. In case of == there might be some difference. The thing is that if you accidentally write = instead of == the compiler will give an error in the first case.

if (threadInfo = NULL) //always false. The compiler will give you a warning at best

if (NULL = threadInfo) //Compiler error

I personally hate that practice and think it's better to write code that can be read in a normal human language, not Yoda language.

Answer 2

These are exact equivalents in terms of logic.

The second is more intutive, when one may see the first is seen as more safe as it doesn't allows writing (threadInfo = NULL) erroneously, leading to a bug.

Answer 3

It's for safety, so you don't accidentally write

threadInfo = NULL

instead of

threadInfo == NULL

For the != there's no need to do this, but it's consistent.

Answer 4

Null is a reference in the memory contains nothing but that nothing have an address to access it.

By that you can make a comparisons behind null to check if a certain object have a value or nothing.

Answer 5

If threadInfo is of a type that overrides operator !=, and the owner of that class has not provided a symmetric free function that handles the case where the two arguments are swapped, there might be a difference (there might also be a need to fire that programmer).

Otherwise, it's a matter of taste. Probably it will be preferred by people who write if(42 == answer) instead of if(answer == 42) -- this protects you from mistyping an assignment operator instead of an equals check. But since modern compilers warn you when you do that, it's debatable whether this approach offers anything.