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

Answer 1

Both do the same thing. It is a style convention that was used to avoid typing mistakes as:

if (value=NULL) // assignment instead of == operator 
{ 
}

Answer 2

No difference, this is so called Yoda coding convention.

if (NULL != threadInfo)

is equivalent to

if (threadInfo != NULL)

and to

if (threadInfo)

Answer 3

There is no difference, EXCEPT that the first option cannot be mistyped as:

if (threadInfo = NULL)
{
    ...
    ...
}

And some people don't know how to use compiler switches to check this (and some compilers don't have them).

Answer 4

There is no difference. The point of writing NULL != ... is that if you instead make a typo and write NULL = ... the code won't compile. If you had ... = NULL it could be a valid assignment and the error could go unnoticed (but most compilers detect this and warn you). Somebody once tried to insert a backdoor into the Linux kernel using this technique.

Also note that most persons don't code like that.

Answer 5

I don't see any difference. However, the practice of writing NULL == someVar would save you from writing NULL = someVar if you forget typing the second =.

Answer 6

This is usually used as a convention, and has no distinct meaning by itself.

Because assignment occurs to lvals, and NULL is unassignable, this protects against the dreaded threadInfo = NULL when you meant threadInfo == NULL or threadInfo != NULL bug.

For more detail, see this wikipedia article section on left-hand comparisons