Vector of objects with a unique_ptr member and custom destructor

user_5692423 0
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Assume the following piece of code to move an object of type A into av (vector):

#include <memory>
#include <vector>

struct A
{
    ~A() {};

    std::unique_ptr<int> an_int;
};


int main(int argc, char* argv[])
{
    std::vector<A> av;
    av.push_back(A());

    return 0;
}

This will fail to compile as is. However if I do not implement the destructor ~A() {}, it will work just fine. The compiler hints at some missing (deleted) copy constructor, but I fail to see where that applies here (as I am only doing move operations)

What is the relationship between the unique_ptr and the custom destructor that makes the code fail to compile? is there a simple work around for this?

Thanks.

c++17 c++
2 Answers

Defining a custom destructor disables generation of the default move constructor and move assignment operator. Thus push_back(const A&) is used, but A does not have copy constructors either since it has unique_ptr member.

The solution is to manually define them as default. But defining the move constructor disables the default constructor:

struct A
{
    ~A() {};
    A()=default;
    A(A&&)=default;
    A& operator=(A&&)=default;
    std::unique_ptr<int> an_int;
   
};
static_assert(std::is_nothrow_move_constructible_v<A>);
static_assert(std::is_nothrow_move_assignable_v<A>);
user_76917290

When you define a destructor the compiler does not generate a move/copy constructor since you just hinted that the construction/destruction of the object is not trivial. See this answer.

user_3909130
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