Re: const char* differences between VS6 & VS8

From:

Tamas Demjen <tdemjen@yahoo.com>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 12 Dec 2008 13:56:26 -0800

Message-ID:

<#NBGKTKXJHA.1268@TK2MSFTNGP04.phx.gbl>

John Keenan wrote:

You probably have an alignment problem. In one compiler the variable is
4 bytes off. The first thing I'd check is that sizeof(MyClass) is the
same in both compilers.

Why would returning c_str() from an inline function cause a problem? I
thought a "C string" (const char*) was a built in type that is well defined
and well behaved.

The std::string implementations may not be binary compatible between the
two compilers. There could be a data alignment mismatch (#pragma pack),
or if you mix debug and release builds and the header file has #ifdef
_DEBUG, that causes a misalignment:

struct Test
{
    int x;
#ifdef _DEBUG
    int debug_value;
#endif
    int y;

    inline int GetY() const { return y; }
};

Anyway, these are just a few examples of what might go wrong. For better
portability, use pure virtual functions, or COM:

struct Interface
{
    virtual const char* GetString() const = 0;
};

struct Impl : public Interface
{
    std::string s;

    virtual const char* GetString() const { return s.c_str(); }
};

Interface classes like that are guaranteed to be 100% portable on the
Windows platform, because COM relies on compatible vtable implementations.

Tom