What is the difference between #define and const? [duplicate]

Question

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What is the difference between using #define and const for creating a constant? Does any have a performance advantage over the other? Naturally I prefer using the const but I'm going to consider the #define if it has suitable advantages.

Answer 1

The #define directive is a preprocessor directive; the preprocessor replaces those macros by their body before the compiler even sees it. 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, performance: Perhaps you're thinking that avoiding the declaration of a variable saves time and space, but with any sensible compiler optimisation levels there will be no difference, as constant values are already substituted and folded at compile time. But you gain the huge advantage of type checking and making your code known to the debugger, so there's really no reason not to use const variables.

Answer 2

#define creates an entity for substitution by the macro pre-processor, which is quite different from a constant because depending on what you define it will or will not be treated as a constant. The contents of a #define can be arbitrarily complex, the classic example is like this:

#define SQR(x) (x)*(x) 

Then later if used:

SQR(2+3*4) 

That would be turned into:

(2+3*4)*(2+3*4)