What is the benefit of using #define to declare a constant ? [CLOSED]

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 cast it, 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.