Can #define replace typedef

[parag_paul]

Is there any particular point that makes typedef different than
#define

 

[WANG Cong]

Is there any particular point that makes typedef different than
#define

For example,

if you use #define to define a new type, like this:

#define foo_t unsigned int *

then,

foo_t a, b;

'a' has type of 'unsigned int *', while 'b' has type of
'unsigned int'.

If you use typedef, you will get the correct behavior.

typedef unsigned int * foo_t;
foo_t a, b;

Both 'a' and 'b' have a pointer type.

 

[parag_paul]

For example,

if you use #define to define a new type, like this:

#define foo_t unsigned int *

then,

foo_t a, b;

'a' has type of 'unsigned int *', while 'b' has type of
'unsigned int'.

If you use typedef, you will get the correct behavior.

typedef unsigned int * foo_t;
foo_t a, b;

Both 'a' and 'b' have a pointer type.

thanks Wang

 

[sg]

#include <iostream>

using namespace std;

typedef unsigned int * foo_t;
//#define foo_t unsigned int*

int main(void)
{
foo_t a, b;
//unsigned int* a, b; // Will not work

unsigned int c = 10;

a = &c;
b = &c;

cout << *a << endl << *b << endl;

return 0;
}

Regards
Sandeep

thanks Wang

I think parag has already solve your doubt. I am giving an example in C
++ for the better understanding

 

[red floyd]

Is there any particular point that makes typedef different than
#define

Yes. #defines do not respect scope. No matter what context you are
in, the #define takes effect. It's pure textual substitution that
occurs before almost every other phase of compilation.

A typedef, on the other hand, is part of the language syntax (as opposed
to the preprocessor). It respects scope.

 

[Juha Nieminen]

For example,

if you use #define to define a new type, like this:

#define foo_t unsigned int *

then,

foo_t a, b;

'a' has type of 'unsigned int *', while 'b' has type of
'unsigned int'.

There are also some types which are basically impossible to implement
as a #define. For instance:

typedef int(*FunctionPointer)(int);

// Usage example:
int foo(FunctionPointer fptr)
{
return fptr(5);
}

Another example:

typedef int color[3];

// Usage example:
color c;
c[0] = 10;

 

[WANG Cong]

There are also some types which are basically impossible to implement
as a #define. For instance:

True, typedef "encapsulates" a new type in syntax, which happens during
compile time, #define does things literally, which happens during pre-
process time, before compiling.