shared_ptr to abstract base class (member variable) is a an undeclared identifier

Question

The following struct should hold a shared_ptr to an abstract base class (Shape). However, when I try to write this, I get error C2065: 'Shape' : undeclared identifier.

So I know that I can't create instances of Shape, only instances of the derived Spheres, Boxes, etc. But why can't I use a shared_ptr to an abstract base class?

#include <memory>  // std::shared_ptr

#include "shape.hpp"

struct Hit {
    // …

    std::shared_ptr<Shape> m_shape;  // Shape that was hit
};

It should be noted that indeed the Shape class is using the Hit struct and vice versa.

@Quentin This is correct. Both files indeed include each other. — kleinfreund, Aug 14 '15 at 16:01

Bathsheba · Accepted Answer · 2015-08-14T16:09:54.427

6

Replace #include "shape.hpp" in the above with the forward declaration

class Shape; (you might need struct Shape; depending on how Shape is declared).

This is possible since class members of type std::shared_ptr can be declared with an incomplete type.

This helps you to untangle circular inclusions. Of course, the source file that defines the functions for struct Hit will need to #include shape.hpp.

edited Aug 14 '15 at 16:09

answered Aug 14 '15 at 16:02

Bathsheba

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In the case of having only a `hit.hpp` and no cpp file (There is only constructors and member declarations), am I leaving `#include "shape.hpp"` or am I introducing a cpp file? – kleinfreund Aug 14 '15 at 16:05
For an easy life, introduce a `.cpp` file. – Bathsheba Aug 14 '15 at 16:05
And sources will be more readable when you have hpp and cpp file pair ... – Robert Wadowski Aug 14 '15 at 16:07

shared_ptr to abstract base class (member variable) is a an undeclared identifier

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