CODEWITHSUNDEEP
c++
jakebird451
jakebird451

Reputation: 2348

Strange instantiation in a class

I am looking over some open source code and they listed some strange instance in a class defined as:

Obj *&obj;

I had no idea that was possible! What does that mean or even do? The pointer and reference symbols should counteract in my opinion. But maybe it does something special I do not know about?

Looking in the code, they use it as if the instance was a pointer. ie: obj->method(). So then why did they include the &?

Upvotes: 1

Views: 70

Answers (4)

justin
justin

Reputation: 104698

That's a reference to a pointer. After all, a pointer is also a value. You might use it to reassign a pointer held by the caller. To illustrate:

bool allocate(t_object*& p) {
  assert(0 == p);
  p = new t_object;
  ...
}

Then:

t_object* obj(0);
if (!allocate(obj)) {...}
// obj now points to something

Some people find it easier to manage/read that its most direct alternative (bool allocate(t_object** p)).

As to why a reference to a pointer was chosen in the program -- there's not enough context, but you can ask yourself if the program would be better if the member were declared t_object** const?

Upvotes: 1

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361462

That is reference to a pointer. Think in this way:

A *pa = new A(); //pa is pointer to A

A * & rpa = pa; //rpa is reference to pa

rpa = nullptr;  //it modifies pa as well

if ( pa == nullptr )
{
     std::cout << "pa is nullptr" << std::endl;
}

It will print "pa is nullptr" on the console.

If you find A * & difficult to read, then you can use typedef as:

typedef A* PA;

PA & rpa = pa; //same as before

Upvotes: 2

Eran Zimmerman Gonen
Eran Zimmerman Gonen

Reputation: 4507

The code like you wrote it won't compile, since it declares a reference to a pointer, but the reference is uninitialized. For example, the following code won't compile:

string *&s;

(unless it's a member in a class), but this will:

string *p;
string *&s = p;

Another possibility, is that this is just a typo and they meant to write Obj *obj; ;)

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258608

It's a reference to a pointer. A pointer is a data-type, why shouldn't you be allowed to have references to one?

It means the same thing as any other type reference - it's an alias for a pointer.

Upvotes: 0

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