Question

I am currently porting a program developed on Linux to Win32. Amongst other problems, I have one that is pretty weird.
A header file contains something like this:

namespace Networking {
  struct  MetaStruct
  {
    int   iDataType;
    int   iDataSize;
    void* pData;
  };

  const int MetaStructSize = sizeof(MetaStruct) - sizeof(MetaStruct::pData);
};

This compiles fine on fine on Linux, but I get this error when compiling for Win32 using VS2010:
Networking.hpp(50): error C2070: '': illegal sizeof operand

I tried adding the Networking:: before MetaStruct but it doesn’t change anything. The weird thing is VS2010 gives me the correct value of the sizeof when I hover it with the mouse, but won’t compile it. Why?

Answer 1

In C++03 There are two forms of sizeof expressions (see ISO/IEC 14882:2003 5.3.3 [expr.sizeof]).

sizeof unary-expression
sizeof ( type-id )

MetaStruct::pData is neither a valid expression (resolving to an object type) nor the name of a type.

You would have to do

sizeof( void* )

or

sizeof MetaStruct().pData

---

Update: Thanks to @hvd who points out that this should actually be legal in C++11 now.

You can now use an id-expression that refers to a non-static member of a class in contexts where it isn’t evaluated. Evidently this isn’t supported by VS2010.

There’s also a new form of sizeof in C++11: sizeof ... ( identifier ) but that’s not relevant here.