Forward declaration cv::Mat

Question

Hi I am trying to forward declarate the cv::Mat class but I cant get it to work. It gives the message field 'frame' has incomplete type.

OpenGlImpl.h

namespace cv {
    class Mat;
}

class OpenGLImpl {

private:
   cv::Mat frame;

};

How should I properly forward declarate this?

Answer 1

You cannot use a forward declaration here. The compiler needs to have the definition of cv::Mat in order for it to be a data member of OpenGLImpl.

If you want to avoid this constraint, you could have OpneGLImpl hold a (smart) pointer to cv::Mat:

#include <memory>

namespace cv {
    class Mat;
}

class OpenGLImpl {

private:
   std::unique_ptr<cv::Mat> frame;

};

You can then instantiate the cv::Mat owned by the unique_ptr in an implementation file.

Note that a reference would also work with a forward declaration, but it is unlikely you need refernce semantics here.

Answer 2

§ 3.9.5

A class that has been declared but not defined, or an array of unknown size or of incomplete element type, is an incompletely-defined object type.43 Incompletely-defined object types and the void types are incomplete types (3.9.1). Objects shall not be defined to have an incomplete type.

struct X; // X is an incomplete type
X* xp;    // OK, xp is a pointer to an incomplete type. 

struct Y
{
   X x;   // ill-formed, X is incomplete type
}     

struct Z
{
   X* xp;   // OK, xp is a pointer to an incomplete type
}     


void foo() {
//  xp++; // ill-formed: X is incomplete
}

struct X { int i; }; // now X is a complete type

X x;           // OK, X is complete type, define an object is fine

void bar() {
  xp = &x;     // OK; type is “pointer to X”
}

void t()
{   
  xp++; // OK: X is complete
}