Mapping a C++ Class to a C Structure

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A C++ class without virtual functions can be mapped to a corresponding C structure, but there are fundamental differences between both data types. The C++ class contains data members and member functions to access and manipulate these data members. The corresponding C structure contains only the data members, but not the member functions contained in the C++ class.

Figure 1 shows the C++ class Class1 and Figure 2 shows the corresponding C structure.

Figure 1. Example of C++ Class without Virtual Functions

class Class1
{
 public:
      int m1;
      int m2;
      int m3;
      f1();
      f2();
      f3();
};

Figure 2. Example of C Structure that Corresponds to C++ Class without Virtual Functions

struct Class1
{
    int m1;
    int m2;
    int m3;
};

To access a C++ class from a C program you need to write your own functions to inspect and manipulate the class data members directly.

Note: While data members in the C++ class can be public, protected, or private, the variables in the corresponding C structure are always publicly accessible. You might eliminate the safeguards built into the C++ language.

You can use C++ operators on this class if you supply your own definitions of these operators in the form of member functions.

When you write your own C++ classes that you want to access from other languages:

Do not use static data members in your class, because they are not part of the C++ object that is passed to the other language.
Do not use virtual functions in your class, because you cannot access the data members because the alignment of the data members between the class and the C structure is different.
Note: By making all data members of a class publicly accessible to programs written in other languages, you might be breaking data encapsulation.