Question

Consider a piece of code like this:

struct B {
    int c;
    B() {
        c = 20;
    }
};
struct A {
    boost::optional<B> m_b;
    void f() {
        B b; 
        this->m_b = b;
    }
};

int main(void) {
    A a;
    a.f();
    cout << a.m_b->c << endl;
}

Apparently, a.m_b still validly exists, after assigning a local variable b to m_b.

This kind of confuses me, because I thought assigning an object to boost::optional<> was just assigning the address that points to the object. Since in my example that object is b, which is a local variable, its address should be invalid after function f() is done.

Then why is a.m_b still alive? It should be pointing to an invalid address.

Answer 1

boost::optional‘s operator= will call B‘s copy constructor. If you insert this into your struct B you can set a breakpoint on it to see what’s going on:

B(B const& other)
{
}

Boost uses a placement new in its implementation, maybe this is what confuses you?