Static, define, and const in C

Question

I\'ve read that static variables are used inside function when one doesn\'t want the variable value to change/initialize each time the function is called. But what about def

Answer 1

The main difference is that with #define you leave the type system. The preprocessor has no notion of type safety, scope etc. So e.g. if you later try to write a loop like

for (int m = 0; m < size; m++) { ... }

you are up to a nasty surprise...

Also if you use #defines, you will only see the value of 30000 when debugging your code, not the name m. Which does not seem to make a big difference in this case, but when using meaningful constant and variable names, it does indeed.

Answer 2

In the toplevel scope static means that the variable (or function) cannot be accessed outside this source file - it won't be made available to the linker, and won't cause any name conflicts when linked in. It has no effect on whether a variable is constant or not - in fact, such variables are often specifically non-constant so that initialization can be cached.

The difference between using const and #define is that the former allows the compiler to type-check your usage of a constant.

Answer 3

When you write const double m=3000; you are telling the compiler to create a symbol m in the object file that can be accessed from other files. The compiler may inline the value of m in the file where it is defined, but the symbol still has to be allocated for the purposes of separate compilation.

When you write #define m 3000 you are just using a syntactic convenience for writing the same constant in several places in the source file.