Macro Definitions

Macro definitions are recognized during preprocessing phase, and macro names are naively replaced by the preprocessor.

Consider following example:

#define Int_Ptr (int *)

Int_Ptr a, b, c;

In the above case, only a will be defined as a pointer pointed to and Integer, whereas b and c are two Integers.

Preprocessor just replace macro names as it sees them, so after preprocessing the above code becomes:

int * a, b, c;

Similar to pointers, array types can't be defined as macros either.

Type Definitions

Type definitions are not preprocessed directives but compiled statements. They literally define a new type. So it can define new pointer and array types which macro definitions cannot.

typedef int* Int_ptr;

Int_Ptr a, b, c;

typedef char Char_Arr[10];

Char_Arr x, y;

The above code will give us 3 Integer pointers and 2 arrays of 10 characters.

One of the biggest differences is that type definitions are subject to the same scope rules are variables, that is a typedef defined typed in a function body will not be recognized outside that function. However, macro names will be replaced everywhere in the source file as long as the preprocessor finds a macro.