Is it possible to use #define inside a function?

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#define is a preprocessor directive: it is used to generate the eventual C++ code before it is handled to the compiler that will generate an executable. Therefore code like:

for(int i = 0; i < 54; i++) {
  #define BUFFER_SIZE 1024
}

is not executed 54 times (at the preprocessor level): the preprocessor simply runs over the for loop (not knowing what a for loop is), sees a define statement, associates 1024 with BUFFER_SIZE and continues. Until it reaches the bottom of the file.

You can write #define everywhere since the preprocessor is not really aware of the program itself.

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