Hello #cpp / #cplusplus Fediverse,

[riley@toot.cat]

Hello #cpp / #cplusplus Fediverse,

If I've got types struct foo and struct bar, which are unrelated by formal class inheritance, is there some way to define a conversion from struct foo * to struct bar * ?)

I can't attach a plain old conversion operator to a pointer type, even if I name it, and none of the C++ compilers that I tried thinks of invoking a reference conversion operator when what's supposed to be implicitly converted, and the expectant receptacle, are nominally pointers.

Is there a way?

[oblomov@sociale.network]

@riley what do you mean by a conversion between pointer types?

[riley@toot.cat]

@oblomov Defining a function/operator that would be implicitly invoked when struct foo * is passed to a function declared to accept a struct bar * that would ideally amount to adding a small constant to the pointer; effectively, there's a struct bar inside a struct foo, and I'd like a struct bar * to be converted to struct foo * by implicitly referring to that inner struct when a pointer to the outer struct is being passed to a function taking a pointer to the inner struct.

[lesley@mastodon.gamedev.place]

@riley @oblomov I think in this particular case (one object has the type of the first member of another object), you are allowed to cast? I've done this trick in C, but I am not entirely sure about C++, though. The standard seems to imply so

https://timsong-cpp.github.io/cppwp/basic.compound#5.3

[oblomov@sociale.network]

@lesley yes, but in the case of @riley that's not the same issue: if foo has-a bar, there's no way to do the casting _implicitly_, as far as a I know. It would be possible with references by defining an implicit constructor of bar from foo, but this won't be applied to pointers.

The only work around I can think of is to define an overloaded function

bar * get_bar(bar * o) { return o; }
bar * get_bar(foo * o) { return &o->bar_element; }

and wrap pointer arguments with that function.

[riley@toot.cat]

@lesley Unfortunately, it's cumbersome to put struct bar first inside the struct foo; for reasons, I'd prefer struct foo to inherit from a third structure that puts some fields in its beginning, and struct bar can not inherit from it.

@oblomov

[riley@toot.cat]

@lesley Also, I'd have to explicitly cast; the cast would not be invoked implicitly by the language.

@oblomov

[oblomov@sociale.network]

@riley @lesley the only way to avoid explicit casts is inheritance. Can you make foo inherit from both whatever you want it to inherit from AND bar? Or is multiple inheritance a no-no in your project?