C++ 底漆 5 版。 虚拟析构函数和移动语义

Question

In C++ primer 5 ed. it is said:

"The fact that a base class needs a virtual destructor has an important indirect impact on the definition of base and derived classes: If a class defines a destructor—even if it uses = default to use the synthesized version—the compiler will not synthesize a move operator for that class (§13.6.2, p. 537)."

So for more understanding I've tried this:

class A
{
public:
    A(){cout << "A::A()\n";}
    virtual ~A(){cout << "A::~A()\n";}
    A& operator = (const A&) {cout << "A::=\n"; return *this;};
    A& operator = (A&&) = default;
};

int main()
{

    A a;
    A a2;
    a2 = std::move(a); // why copy-assignment operator is not called here. Normally a will be "moved" through copy-ctor (moving through working copy-ctor)


    std::cout << std::endl;
}

• So according to the text above in the book class A has a virtual destructor thus the compiler won't synthesize a move-ctor/assignment operations But in main I've tried to move an object of type class A so normally move here uses copy-assignment instead of move-assignment but copy-assignment is not called here?!

• So when a vairtual dtor causes a default operation be not synthesized? Thank you!

Answer 1

Your textbook is right, the compiler won't automatically synthesize a move operator. In the code sample you force the compiler to generate one by setting it to default, this can only fail if the move operator is implicitly deleted . Which is something else than it being implicitly declared .

Criteria for implicit deletion :

• A has a non-static data member that is const
• A has a non-static data member of a reference type
• A has a non-static data member that cannot be move-assigned (has deleted, inaccessible, or ambiguous move assignment operator)
• A has direct or virtual base class that cannot be move-assigned (has deleted, inaccessible, or ambiguous move assignment operator)
• A has a non-static data member or a direct or virtual base without a move assignment operator that is not trivially copyable
• A has a direct or indirect virtual base class

Criteria for implicit declaration :

• there are no user-declared copy constructors;
• there are no user-declared move constructors;
• there are no user-declared copy assignment operators;
• there are no user-declared destructors;
• the implicitly-declared move assignment operator would not be defined as deleted

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