CODEWITHSUNDEEP
c++pointerstypedef
user2653125
user2653125

Reputation: 185

Multiple Pointer Declaration With typedef

In C++, if we want to declare multiple pointers, we would do something like this: int *a, *b, *c; where we would have to put an asterisk * in front of each of them. if i write this code: typedef int* ptr; ptr a,b,c;? Will they all be pointers, or just a?

Upvotes: 5

Views: 880

Answers (4)

Rontogiannis Aristofanis
Rontogiannis Aristofanis

Reputation: 9063

Hmmm, interesting... Lets see. Say I don't know many things about typedefs, what do I do?

Answer: I test it by myself. Below there is a code which answers your question:

#include <stdio.h>

typedef int* pointer_t;
pointer_t a1, b1, c1;
int* a2, b2, c2;

int main() {
  a1 = new int; // ok
  b1 = new int; // ok
  c1 = new int; // ok
  a2 = new int; // ok
  b2 = new int; // compile-time error
  c2 = new int; // compile-time error

  delete a1; // ok
  delete b1; // ok
  delete c1; // ok
  delete a2; // ok
  delete b2; // compile-time error
  delete c2; // compile-time error

  return 0;
}

Conclusion: Both a1, b1, and c1 will be pointers, but only a2 will be a pointer.

Upvotes: 3

molbdnilo
molbdnilo

Reputation: 66371

No, typedef isn't just a matter of text substitution (like a macro would be). 

typedef int* ptr;

introduces a new name, "ptr", for the type int*.

If you write

ptr a, b, c;

all of a, b, and c will have the same type, int*.

Note that

const ptr p;

likewise isn't the same as

const int* p;

Since ptr is a pointer type, the const applies to the pointer; the equivalent is

int* const p;

Upvotes: 5

Manu343726
Manu343726

Reputation: 14174

Yes, they will be pointers:

typedef int* pointer_to_int;

int main()
{
    int a , b , c;
    pointer_to_int ptr_a , ptr_b , ptr_c;

    ptr_a = &a;
    ptr_b = &b;
    ptr_c = &c;

    *ptr_a = 1;
    *ptr_b = 2;
    *ptr_c = 3;

    std::cout << a << " " << b << " " << c;
}

The output is:

1 2 3

Thats because typedef defines type aliases. On the other hand, if you could use C++11, I recommend you to use using aliases instead of typedef, because its syntax is much clear:

using ptr_to_int = int*;

and also works with template aliases:

template<typename T>
using vector_t = std::vector<T>;

Upvotes: 1

Paul Evans
Paul Evans

Reputation: 27577

They will all be a pointers, the typedef is int*.

Upvotes: 0

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