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What is the point of #define in C++? I've only seen examples where it's used in place of a "magic number" but I don't see the point in just giving that value to a variable instead. c++ - Why use #define instead of a variable - Stack Overflow The #define directive is a preprocessor directive; the preprocessor replaces those macros by their body before the compiler even sees it.
Understanding the Context
Think of it as an automatic search and replace of your source code. A const variable declaration declares an actual variable in the language, which you can use... well, like a real variable: take its address, pass it around, use it, cast/convert it, etc. Oh ...
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In other words, when the compiler starts building your code, no #define statements or anything like that is left. A good way to understand what the preprocessor does to your code is to get hold of the preprocessed output and look at it. c++ - What does ## in a #define mean? - Stack Overflow The question is if users can define new macros in a macro, not if they can use macros in macros. Macros (created with #define) are always replaced as written, and can have double-evaluation problems.
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inline on the other hand, is purely advisory - the compiler is free to ignore it. Under the C99 standard, an inline function can also have external linkage, creating a function definition which can be linked against. As far as I know, what you're trying to do (use if statement and then return a value from a macro) isn't possible in ISO C... but it is somewhat possible with statement expressions (GNU extension). Since #define s are essentially just fancy text find-and-replace, you have to be really careful about how they're expanded. I've found that this works on gcc and clang by default: