#ifdef and #ifndef and #endif

Question

So Could you tell please What are Uses of this three in c++ why we use this
Can you explain the syntax

Answer 1

We use them for safety, to make sure that we will not define the same thing twice using preprocessor, or if we want to do something if something is already defined as "#ifndef" is just "if not defined", and "#ifdef" is for "if defined"
For example:
#ifndef PI
#define PI 3.14
#endif
//____________
#ifdef PI
#undef PI //undefining PI
#define PI 3
#endif
We can also use them to do something same as with "if" statement.
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