The differences between if else and #if #else #endif

Question

I am confused between the if/else and #if/#else/#endif constructs.

1. What are the differences betwee

Answer 1

Can I ask what's the differences between them?

#if #else and #endif are instructions to the compiler, to only compile the code between them, if a compilation level condition (like a macro being defined or having a certain value) is satisfied.

if and else are parts of the compiled algorithm.

What kind of specific situations for me to choose each of them?

The pre-compilation conditions are used to disable parts of the code, in situations where they make no sense (like calls to Windows-specific APIs, when compiling under Linux). They are key to developing cross-platform code (for example).