Как в C++ сделать шаблонный абстрактный базовый класс ковариантным с его параметром шаблона? ⇐ C++

[Гость]

Я знаю это

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std::shared_ptr

et al emulate covariance by having templated constructors.
But let's say I have an abstract base class that's a simple accessor:

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template 
class Reader {
public:
virtual const T& Get() const = 0;
};

Then if I have a Base and Derived class (it doesn't matter what they do):

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class Base {};
class Derived : public Base {};

It should be safe to receive a

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std::unique_ptr

and pass it around as a

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Reader

. I think this should work:

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void Frobnicate(const Reader& reader) {}

void Run() {
std::unique_ptr reader = GetReaderSomehow();
Frobnicate(*reader);
}

But if you do this you'll get an error about

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Reader

and

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Reader

not being convertible (since, fair enough, they don't inherit from one another):

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:17:6: note: candidate function not viable: no known conversion from 'Reader' to 'const Reader' for 1st argument
17 | void Frobnicate(const Reader& reader) {}

Since

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Reader

is abstract, I can't just add a converting constructor to it because it will try to create an instance of Reader behind the scenes to pass as the const reference. Similarly if I try to take a

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std::unique_ptr&

as the argument to Frobnicate, the non-convertibility makes the unique pointers non-convertible.
So, how can I do this?


Источник: https://stackoverflow.com/questions/780 ... ts-templat