Using placement new to update a reference member?

Question

Is the following code legal in C++?

template
class Foo {
public:
    Foo(T& v) : v_(v) {}

private:
    T& v_;
};

int a = 10;
Foo         


        

Answer 1

It may be legal, but it's incredibly poor style. The argument to placement new is a void*, so you're telling C++ to reinterpret_cast the address of f as a void*, then using that as the location to construct something new - overwriting the original f.

Basically, don't do that.