在MSVC上的数组初始化期间调用了没有复制或移动构造函数的析构函数

Question

I have tried this on MSVC versions 19.10.25019.0 and 19.11.25547.0 in debug and release builds and get the same result.

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The following program prints 0 1 2 3 4 5 . I expected either

• no output, or
• a combination of move or copy constructor calls with the destructor calls.

Instead, it seems the destructor was called for 6 of the 7 elements and no copy or move constructor calls were made.

#include <iostream>

struct MyChar {
    MyChar(char c) : c{c}{}
    MyChar() = default;

    MyChar(const MyChar&) { std::cout << "copy"; };
    MyChar& operator=(const MyChar&) { std::cout << "assign"; return *this; };
    MyChar(MyChar&&) { std::cout << "move"; };
    MyChar& operator=(const MyChar&&) { std::cout << "move assign"; return *this; };

    ~MyChar() { std::cout << cnt++ << '\t'; }

    char c{'H'};
    static int cnt;
};

int MyChar::cnt{};

int main()
{
    auto arr1 = new MyChar[7]{'D'};
}

Why is the destructor being called 6 times without delete (or without the compiler initializing via a copy or a move)?

Answer 1

In playing around with this in the VC compiler, and inspecting the generated assembly code, this appears to be a compiler bug related to the MyChar() = default constructor. Replacing it with MyChar() { } gets rid of the unexpected destructor calls.

By adding some additional code into the destructor, the objects being destroyed are the default-initialized members of arr1 . Adding in a call to delete [] arr1 , along with including the address of the object being destroyed in the destructor, shows that the first element is destroyed once, while the other 6 are destroyed twice - once when arr1 is constructed, and again when the delete call is made.

This should be reported to Microsoft. It occurs in both VC2015 and VC2017.