Disambiguate the diamondproblem in c++ to access the top level member

Question

First I would like to acknowledge that there are many questions with similar titles and subject matter. I'm fairly certain mine is unique from them.

Given the code

struct Top {
    int get() {
        return 0;
    }
};

struct Mid1 : Top {
    int get() {
        return 1;
    }
};

struct Mid2 : Top {
    int get() {
        return 2;
    }
};

struct Bottom : Mid1, Mid2 {

};

int main(int argc, char ** argv) {
    Bottom b;
    std::cout << b.Mid1::get();
    std::cout << b.Mid2::get()
    std::cout << b.Top::get();
}

I get an error on the line trying to access Top::get() with the error (using gcc mingw-w64)

 error: 'Top' is an ambiguous base of 'Bottom'
 std::cout << b.Top::get();

I get an error. How would one signal to the compiler to call Top::geton a specific inheritance path in this case?

It's unusual to see a diamond problem that doesn't feature the keyword `virtual` — Tim Randall, Aug 23 '18 at 20:06
while using virtual is probably better code, using the virtual keyword does not solve this problem — rtpax, Aug 23 '18 at 20:10
@NathanOliver The one the user specifies. basically I'm asking how to specify — rtpax, Aug 23 '18 at 20:11
As I understand the problem, `Top::get()` can only be called on an object of class `Bottom` because `Bottom` is ultimately derived from `Top`. The problem is, how is it derived? There are two paths leading from the parent class to the grandchild, and no way to decide which path to take. And I don't know of a way to tell the compiler to take either one . — Tim Randall, Aug 23 '18 at 20:11
@Eugene I think I'm somewhat lacking in my knowledge of subtle differences between the two. I'll have to do my research. If the difference between the two allows an answer feel free to post — rtpax, Aug 23 '18 at 20:19
@TimRandall "*I don't know of a way to tell the compiler to take either [path]*" - see my answer — Remy Lebeau, Aug 23 '18 at 20:21

Remy Lebeau · Accepted Answer · 2018-08-23T20:24:50.227

3

Bottom is a derived class that is not using virtual inheritance, so it has 2 distinct instances of Top - one inherited from Mid1, and another from Mid2. As such, you must explicitly tell the compiler which instance of Top you want to call get() from, eg:

std::cout << b.Mid1::get(); // OK
std::cout << b.Mid2::get(); // OK
std::cout << b.Top::get(); // ERROR!
std::cout << b.Mid1::Top::get(); // ERROR!
std::cout << b.Mid2::Top::get(); // ERROR!
std::cout << static_cast<Mid1&>(b).Top::get(); // OK
std::cout << static_cast<Mid2&>(b).Top::get(); // OK

Live Demo

edited Aug 23 '18 at 20:24

answered Aug 23 '18 at 20:16

Remy Lebeau

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This does exactly what I was looking for in an answer. A bit ugly, but I wouldn't really expect much else. – rtpax Aug 23 '18 at 20:18

Eugene · Answer 2 · 2018-08-23T20:37:00.963

0

Is your intention to have 2 copies of Top inside Bottom? If it is, then use the answer from Remy Lebeau. But if you instead want to have a single copy of Top inside Bottom, then use virtual inheritance:

struct Mid1 : virtual Top {...}
struct Mid2 : virtual Top {...}

In this case, your original code

std::cout << b.Top::get();

will compile.

edited Aug 23 '18 at 20:37

answered Aug 23 '18 at 20:26

Eugene

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Disambiguate the diamondproblem in c++ to access the top level member

2 Answers2