CODEWITHSUNDEEP
c++heap-memorystack-memory
Cameron
Cameron

Reputation: 671

Heap allocation of abstract class

I am wondering why I cannot initialize an array of abstract objects to the stack but not the heap.

Here is some C++ code similar to mine that fails on the last line. I'm mainly just curious with the reasons behind this problem dealing with the heap vs. the stack. Thanks in advance!

#define ARRAY_SIZE 10

class Obj {
public:
    virtual void fn() =0;
};

class Sub : public Obj {
public:
    void fn() {
        // ...
    }
};

Obj * o1_array[ARRAY_SIZE];
Obj * o2_array = new Obj[ARRAY_SIZE]; // Compiler Error

Upvotes: 0

Views: 619

Answers (1)

juanchopanza
juanchopanza

Reputation: 227508

You are doing two very different things. In the "stack" case,

Obj * o1_array[ARRAY_SIZE];

you are initializing an array of pointers. You don't need a complete or non-abstract type for this. In the "heap" case,

Obj * o2_array = new Obj[ARRAY_SIZE];

you are initializing an array of objects on the RHS and assigning the address of the first element to a pointer in the LHS. You need a complete, non-abstract type to build the array. If you want a dynamically-sized collection of pointers, a good bet is to use a container of pointers (assuming something else takes care of the management of the pointees' lifetime) or a container of smart pointers (where the smartness of the pointer decides who manages the pointee).

Non-managing:

std::vector<Obj*> o1_array(ARRAY_SIZE);

Managing (unique ownership):

std::vector<std::unique_ptr<Obj>> o1_array(ARRAY_SIZE);

For more options on containers managing "pointers", see the boost pointer container library.

Upvotes: 6

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