Is it safe to inherit a C struct in C++ and pass it back into a C library?

Question

I want to, basically, inherit a C struct in C++ (while literally). I have:

struct foo { // C-side definition

    int sz;

    /* whatever */

    // no virtual destructor, special mechanism

};



class cxx_class { 

    /* something here */

    // no virtual destructor, no need

};



class derived : public foo /*, public cxx_class */ {

    /* some other stuff here */

};

derived

instances (in form of

foo*

) will be passed back to the external library which only knows and uses the

foo

part of

derived

(of course). But the problem is that (I assume) the library uses only c-style cast, which are equivalent to

reinterpret_cast

in C++, the

foo

in

derived

must be at the beginning of the memory block.

I wonder if this is defined behavior and does standard guarantee this?

In other words:

Sometimes I use

static_cast

when I know which subclass it actually is. Is it guaranteed that

static_cast

always gives the same result as

reinterpret_cast

?

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Answer 1

derived instances (in form of foo*) will be passed back to the external library which only knows and uses the foo part of derived (of course).

If your functions take a foo*, it doesn't matter. Even if constructing your types in such a way that foo is not placed at the beginning of derived, the conversion from derived* to foo* happens in the C++ code which does know about the type layout. The external function then doesn't need to worry about it.

Is it safe to inherit a C struct in C++ and pass it back into a C library?

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