使用 unique_ptr 时调用隐式删除的复制构造函数 c++ 17

Question

I have following setup:

class A {
public:
    constexpr A() = default;
    virtual ~A() = default;

    virtual void print() const noexcept = 0;
    virtual std::unique_ptr<A> clone() const noexcept = 0;
};

class B : public A {
public:
    constexpr B() {};
    B(const A& a1, const A& a2) : ptr1(a1.clone()), ptr2(a2.clone()) {}

    virtual ~B() = default;

    virtual void print() const noexcept override {
        std::cout << "In B\n";
    }

    void printPtrs() const noexcept {
        ptr1->print();
        ptr2->print();
    }

    virtual std::unique_ptr<A> clone() const noexcept override {
        return std::make_unique<B>(*this);
    }

private:
    std::unique_ptr<A> ptr1;
    std::unique_ptr<A> ptr2;
};

class C : public A {
public:
    constexpr C() = default;
    virtual ~C() = default;

    virtual void print() const noexcept override {
        std::cout << "In C\n";
    }

    virtual std::unique_ptr<A> clone() const noexcept override {
        return std::make_unique<C>(*this);
    }
};

===== MAIN =============
#include "MyClass.h"

#include <vector>
#include <string>
#include <memory>

using namespace std;

int main()
{
    C c;
    B b;
    B B_container(b, c);
    B_container.printPtrs();

    return 0;
}

I used the 'clone()' method taking the suggestion from this issue: Abstract class and unique pointer

I am getting this error: call to implicitly-deleted copy constructor of 'B' I am really struggling with this, I don't understand why this copy ctor being called here? Any work around?

Answer 1

B::clone attempts to copy a B object, but B is non-copyable due to its std::unique_ptr members.

Since you have a constructor that accepts two references to A objects and clones them, you could use that instead:

virtual std::unique_ptr<A> clone() const noexcept override {
    return std::make_unique<B>(*ptr1, *ptr2);
}

This will create a new B object that holds pointers to copies of the A objects pointed to by this object's unique_ptr members instead of attempting to copy the unique_ptr s themselves, which you cannot do.