custom constructors for built-in types in c++

Question

Suppose I have defined a class called Entity which has a data member which is a pointer to int. So for example

class Entity {
public:
    int* ptr;
    // other stuff
};

Is there a way to give the type int* a constructor which takes an Entity object? If I have an Entity object called my_entity, I'd like to be able to do something like this

int* p(my_entity);

or this

int* p = my_entity;

which would require the compiler implicitly call a constructor for int* that takes an Entity.

Is this possible? (I know I could define a public get_pointer() method in the Entity class and do something like int* p = my_entity.get_pointer(); but this seems clunky.)

Answer 1

Well, there is no constructor for a basic pointer - in the sense that there is no function implicitly called in order to initialise the pointer.

The closest you can come is to use a user-defined conversion operator function

class Entity
{
     public:

          operator int *();
};

Entity::operator int *()
{
     // return something appropriate that is of type int *
}


//   sample usage in a function somewhere

int *p = some_entity;    // implicitly conversion that calls the operator int *()

int *another_p = static_cast<int *>(some_entity);    //  explicit conversion

int *yet_another_p = some_entity.operator int *();

There are variants of this, depending on what form of const qualification is needed (e.g. if the operator function doesn't change the object it acts on, it should be const and may be defined as operator const int *()).

It is necessary to ensure that the pointer returned by the operator function is treated appropriately. If the user defined operator function returns a member of some_entity, it cannot be used once some_entity ceases to exist. Similarly, if it uses dynamic memory allocation (e.g. return the result of a new expression) the caller must explicitly release that memory to avoid a memory leak.

Answer 2

You cant make a constructor for primitive types. Also there is no need to do this using constructor, you could just use a getter as you already mentioned, or a function to which you pass an Entity and int and store the int to Entity.

Also you can use a user-defined conversion as @Some programmer dude said and do something like:

int *p = (int*)my_entity;