Question

I try to access in a C program data received from a C++ library which has been originally designed with structure inheritance:

example:

// C++ data structures
typedef struct _Base
{
public:
    int id;
    wchar_t* name;
} Base;


typedef struct _Struct1 : Base
{
public:
    int valueCount;
} Struct1;


typedef struct _Struct2 : Base
{
public:
    int parentID;
    int amount;
} Struct2;

I tried using the following data structures in C for mapping.

typedef struct _Base
{
    int id;
    wchar_t* name;
} Base;


typedef struct _Struct1
{
    // Base struct data
    int id;
    wchar_t* name;

    int valueCount;
} Struct1;


typedef struct _Struct2
{
    // Base struct data
    int id;
    wchar_t* name;

    int parentID;
    int amount;
} Struct2;

But printing the data, it looks like I get wrong values.

Am I missing something, any reference on how C++ represents inherited structure internally?

Thanks in advance!

Answer 1

The C++11 rules on PODs (What are Aggregates and PODs and how/why are they special?) specify that don’t allow mixing concrete base classes with data members, but in practice for most compilers having a single POD base class is equivalent to encapsulating that class as the first member.

Try specifying your C structs encapsulating the base struct:

typedef struct _Struct1
{
    Base base;
    int valueCount;
} Struct1;

Note that this won’t work if the C++ classes are non-POD (e.g. have virtual methods).