Lambda capture [*this] in class get segment fault

Question

In C++20, [*this] was allowed. So I tried following code:

class A
{
public:
    int x = 5;
    function<int(int)>fun = [*this](int y) mutable
    {
        return x;
    };
};
int main() {
    int x;
    A a{};
    cout<<a.fun(3)<<endl;
    cout<<a.x<<endl;
    return 0;
}

I tried to capture [*this] in class but got segmentation fault; however, the following methods are possible.

class A
{
    int x = 5;
    function<int(int)>fun = [this](int y)
    {
        return this->x + y;
    };
};

class A
{
    int x = 5;
    function<int(int)>fun = [=](int y)
    {
        return this->x + y;
    };
};

class A
{
    int x = 5;
    function<int(int)>fun = [&](int y)
    {
        return this->x + y;
    };
};

Can someone explain this for me?

Answer 1

You're making a copy of an unitialized std::function object.

When an A object is constructed, it needs to make a copy of itself for the lambda that will be used to initialize fun to capture. That copy needs to copy fun, but fun hasn't been initialized yet, and so attempting to make a copy of it results in undefined behavior.

---

Your other examples are fine, because none of them make a copy of *this, and therefore never try to copy its uninitialized fun member.