Offsetof macro with C++/CLI

Question

The offsetof macro seems not to work under C++/CLI.

This works fine in unmanaged C++, but throws "error C2275: 'Entity' :illegal use of this type as an expression" error in CLI.

struct Property{
     char* label;
     PropertyTypes type;
     unsigned int member_offset;
     unsigned int position;
     unsigned char bit_offset;
};

struct Entity{
     ...
     bool transparent;
     ...
};

Property property = {"Transparent",  
       TYPE_BOOL,    
       offsetof(Entity, transparent), 
       0, 
       0}; // C2275 HERE

Does CLI have some replacement?

Answer 1

You will need to provide the type of the object you are assigning to. Looks like there is some type-mismatch for the member in question.

See this for sample usage.

Answer 2

Just a shot in the dark and without a chance to double-check this - should

offsetof(Entity, transparent),

perhaps rather read

 offsetof( struct Entity, transparent ),

???

Answer 3

Standard C++ already has an alternative; &Entity::transparent. You'll probably want to use templates when redesigning the Propery class. The type of a pointer-to-member is non-trivial.

Answer 4

My guess would be that the compiler message boils down to: "offsetof" is not a known macro and if it was a function its parameters must not contain a typename.

Edit: As somebody pointed out in the comments, offsetof is actually part of the std lib. So what's missing is probably just

#include <cstddef>

Alternatively, you can use this macro implementation (taken from Win32/MFC headers):

#ifdef _WIN64
    #define OFFSET_OF( s, m )\
      (size_t)((ptrdiff_t)&reinterpret_cast<const volatile char&>((((s*)0)->m)) )
#else
    #define OFFSET_OF( s, m )\
      (size_t)&reinterpret_cast<const volatile char&>((((s*)0)->m))
#endif