Pointers: initialisation vs. declaration

Question

I am a C++ noob and I am quite sure this is a stupid question, but I just do not quite understand why an error arises (does not arise) from the following code:

#include <iostream>
using namespace std;


int main() 
{
int a,*test; 

*test = &a;  // this error is clear to me, since an address cannot be 
             // asigned to an integer  


*(test = &a); // this works, which is also clear
return 0;
}

But why does this work too?

#include <iostream>
using namespace std;

int main() 
{
int a, *test= &a;  // Why no error here?, is this to be read as:
                   // *(test=&a),too? If this is the case, why is the 
                   // priority of * here lower than in the code above?

return 0;
}

Answer 1

The fundamental difference between those two lines

*test= &a; // 1
int a, *test= &a; // 2

is that the first is an expression, consisting of operator calls with the known precedence rules:

       operator=
          /\
        /    \
      /        \
operator*    operator&
  |             | 
 test           a

whereas the second is a variable declaration and initialization, and equivalent to the declaration of int a; followed by:

   int*     test    =  &a
// ^^        ^^        ^^
//type    variable    expression giving
//          name        initial value

Neither operator* nor operator= is even used in the second line.

The meaning of the tokens * and = (and & as well as ,) is dependent on the context in which they appear: inside of an expression they stand for the corresponding operators, but in a declaration * usually appears as part of the type (meaning "pointer to") and = is used to mark the beginning of the (copy) initialization expression (, separates multiple declarations, & as "reference to" is also part of the type).

Answer 2

int a, *test= &a;

is equivalent of:

int a;
int* test = &a;

and perfectly valid as you initialize test which has a type of pointer to int with an address of variable a which has a type of int.

Answer 3

You're confusing two uses for *.

In your first example, you're using it to dereference a pointer. In the second example, you're using it to declare a "pointer to int".

So, when you use * in a declaration, it's there to say that you're declaring a pointer.