具有void（0）的C ++指令

Question

What is the purpose of having a directive like below?

#define TEST_CONDITION(con) !(con) ? (void)0:

In particular, I see this called at the start of other directives.

For example,

#define OTHER_CONDITION(..)
  TEST_CONDITION(someFunction)
  ANOTHER_DIRECTIVE(...)

Doesn't TEST_CONDITION just no-op or a boolean is returned that isn't used in these cases?

Answer 1

Expand the macro, and it becomes clearer. I'll also use some formatting to keep the code readable, and I assume that the lack of some essential escape characters is not meant to be part of the example. OTHER_CONDITION becomes:

!(someFunction)
    ? (void)0
    : ANOTHER_DIRECTIVE(...)

So, the expression someFunction is executed, and if it is true , then ANOTHER_DIRECTIVE(...) (or whatever it expands to) is executed. Otherwise nothing is executed.

---

Simpler way to write OTHER_CONDITION could be:

#define OTHER_CONDITION(..) if(someFunction) ANOTHER_DIRECTIVE(...)

This simplification lacks some restrictions that TEST_CONDITION provides:

• TEST_CONDITION makes it impossible to append an else branch.
• TEST_CONDITION makes it ill-formed to use a non-void ANOTHER_DIRECTIVE(...) .