如何从类成员函数返回指针，例如 size_t * class :: function(); 并使用类析构函数 ~size_t*class::function();

Question

I am at the point learning classes/destructors in c++ and the need to return a pointer from a class member:

size_t * classname :: function();

but it doesn't work.

My logic is to declare a class variable eg classname * p_p = classfunction(data); that should access the class member:

size_t * classname :: classfunction(data)
{
    //... do something ... 
    new p_p;

    return p_p;
}

So the pointer address in theory gets returned to the main() variable p_p each time the member function of the class gets called. Or should but doesn't and the program crashes somehow but not even sure on which point.

There is no compiler warning or error and the debugger doesn't stops anywhere and I find nothing on returning a pointer from a class member function at all nor that it isn't allowed or something.

Also if there IS a syntax to return a pointer from a class member function I would need to have a syntax for delete the " new p_p ".

So my question is: Should it work and how would I get this running or why is that maybe it is not working or forbidden? In my logic it should be a proper way but I may be wrong somehow and classes doesn't support this function completely.

EDIT:

Thanks to your answers and comments I got the pointer returned from the class member. (Also changed size_t to int since its just a pointer.) Like you suggested in the comments I added a minimal reproducible example:

#include <iostream>

using namespace std;
//########## class without ~ destructor ##########
class classname
{
public:
    int *ptr;
    int *classfunction(int);
    void delete_classfunction(int*);
};


void classname::delete_classfunction(int*ptr)
{
    delete[] ptr;
    ptr = nullptr;
}

int *classname :: classfunction(int value)
{
    int* ptr = new int[value]; //no delete?
    ptr[0] = 5;
    return ptr;
}
//########## class with ~destructor ##########
class classname_c
{
public:
    int *ptr;
    int value;

    *classname_c(int );
    void print(int*);
    ~classname_c();

};

void classname_c::print(int* ptr)
{
    cout << ptr[0] << " shows value" << endl;
}

*classname_c::classname_c(int value)
{
    int* ptr = new int[value];
    ptr[0] = value;
} /*Brings warning: control reaches end of non-void function [-Wreturn-type]
   48 | }
      | ^*/


classname_c::~classname_c ()
{
    cout << ptr[0] << endl;
    delete[] ptr;
    ptr = nullptr;
    cout << ptr[0] << endl;
}

int main()
{   //class and deleting it myself calling the function delte
    int value = 3;
    int *ptr;
    classname call;
    ptr = call.classfunction(value); //create new ptr
    cout << ptr[0] << " shows value" << endl;
    call.delete_classfunction(ptr); //free memory
    cout << ptr[0] << " shows value succ. deleted" << endl;

    //class with destructor
    classname_c dest(value);
    dest.print(ptr);
    cout << ptr[0] << " shows value succ. deleted" << endl; //its not!


    return 0;
}

brings the following output:

5 shows value
0 shows value succ. deleted //How it should be
3 shows value
3 shows value succ. deleted  //but its not, why? What did I do wrong?
Press <RETURN> to close this window...

Now I am not sure if/when the ~destructor is working or how I can test if I created the destructor right, because the ptr value is not deleted. Also can I fix the

warning: control reaches end of non-void function [-Wreturn-type]
   48 | }
      | ^

Answer 1

It is perfectly fine to return a pointer from a member function, although returning a pointer to an allocated size_t seem a bit overkill.

What you want is probably something like this:

#include <iostream>

class cls
{
public:
    size_t *ptr_func();
};

size_t *cls::ptr_func()
{
    size_t *p = new size_t(42);

    return p;
}

int main()
{
    cls c;
    size_t *p = c.ptr_func();

    std::cout << (void *)p << '\n';
    std::cout << *p;

    delete p;
}

Answer 2

This may not answer your question but it might illustrate why this is not working how you expect. When you call a member function of a class, ie classname::function() , you need to have an instance of that class on which to call it. Something which has been defined like

class classname {
public:
    classname() {ptr = new int(0);}
    size_t* function();
    ~classname();

    int* ptr;
};

size_t* classname::classfunction() {
    size_t* ptr = new size_t();
    return ptr;
}

Cannot be called by

classname * p_p = classfunction();

because classfunction() is not being called on an instance of classname and because classname is not size_t so you can't assign classname to the return from a function which returns type size_t unless a cast from one to the other has been explicitly defined (same goes for classname* and size_t* ). You can do something like this

classname cls;
size_t* ptr = cls.classfunction();

Also note that destructors are not used for member functions, only for classes, so the syntax ~size_t*class::function() doesn't really make sense.

If you are trying to get a pointer to a class instance you can simply do

classname * ptr = new classname();

and put your //... do something ... in a constructor.

Edit

The use of a destructor is to perform the

[...] necessary cleanup needed by a class when its lifetime ends.

This means that the destructor is called when the instance of said class goes out of scope or is manually deleted. So when you have a class defined as above, with the destructor

classname::~classname() {
    delete ptr;
}

This means that ptr will be deleted when the instance of classname reaches the end of its "lifetime". Eg

int main() {
   classname* cls = new classname();
   // cls->ptr == int(0)
   delete cls; // Calls ~classname()
   // cls->ptr == NULL
}

The same is true for stack allocation ( classname cls(); ) and in either case the destructor would be called automatically at the end of main (if that instance had not already been manually deleted).

Now what you can't do is delete a pointer that was allocated outside the class instance - if you want to be able to control a pointer like that while still having it accessible from outside the class you can make it a public member, as I did in the edited class declaration. This allows you to access the pointer from outside the class and then still delete it with the destructor,

int main() {
    classname* cls = new classname();
    // cls->ptr == 0
    cls->ptr = 3;
    // cls->ptr == 3
    delete cls;
    // cls->ptr == NULL
}

Hopefully that offers some clarity.

---

The warning is because of the constructor

*classname_c::classname_c(int value) {//...}

.... don't do this until you really know what you're doing, if you want a pointer to an instance of the class the constructor should be

classname_c::classname_c(int value) {//...}

and you should create the instance with

classname_c* cls = new classname_c(value);