C++ return different objects

Question

i have a big problem.. I wonna select the Storage Service via a wrapper class. The returning value must be an object within the storage service class. I pasted my current approa

Answer 1

The problem you are facing is the following: Consider your function

auto test(int select) {
    if (select == 1) {
        return new SQL(); 
    } else {
        return new REDIS();
    }
}

If you trying to evaluate test(1) this expands to

auto test(int select) {
    if (true) {
        return new SQL(); 
    } else {
        return new REDIS();
    }
}

which results in a type error!

I show you three workarounds for your problem:

1. Function template and if constexpr

Make test a function template and check for the correct type using the C++17 feature if constexpr:

template
auto test() {
    if constexpr(std::is_same::value) {
        return new SQL();
    } else {
        return new REDIS();
    }
}

Use it in main() like this:

int main(){
    controller cont;
    auto schnitzel = cont.test();
    auto mitzel = cont.test();
    std::cout << schnitzel->UserhasSurName() << std::endl;
    std::cout << mitzel->UserhasSurName() << std::endl;
}

2. Function template and std::unique_ptr

If you want to avoid using the if constexpr you can simply return an instance of std::unique_ptr instead of a raw pointer. This is the preferred way to do:

template
auto test() {
    return std::unique_ptr(new T);
}

Alternatively you can just return std::make_unique().

3. Returning an instance of the base class

This is is most obvious solution to avoid the type error: Just return an instance of the base class. As above a solution using smart pointers is preferred here:

std::unique_ptr test(const int select) {
    if (select == 1) {
        return std::make_unique();
    } else {
        return std::make_unique();
    }
}

If you really want to avoid using smart pointers just use raw ones like this:

StorageTemplate* test(const int select) {
    if (select == 1) {
        return new SQL();
    } else {
        return new REDIS();
    }
}