Black Moses Black Moses - 2 months ago 12
C++ Question

Why GCC 5.3.0 gives warning when binding reference to "this" pointer

Here is the minimal example:

class A
{
A* const& this_ref;
public:
A() : this_ref(this) {}
};


GCC 5.3.0 gives warning:


warning: a temporary bound to 'A::this_ref' only persists until the
constructor exits [-Wextra]
A() : this_ref(this) {}


Is
this
a temporary then? What the... MSVC 2015 is silent about this, and referring to class members by
this_ref->member
outside the constructor in my case gives expected behaviour (but might be just a case of UB, not sure).




EDIT:

Note this question extends one linked as possible duplicate, because it's not generic question about way to create such reference, but about warning GCC (and possible other compilers other than MSVC) produces when creating one.

Answer

You are creating a dangling reference. Your code is no different from this code:

struct X
{
    const int & r;
    X() : r(5) {}
};     // ^^^^ dangles

There is no "object" called this. this is a keyword, and when used as an expression, it is a prvalue (a temporary) containing the address of the current instance.

Here's another example of the creation of a similarly dangling reference from something that looks like an object but isn't:

struct Y
{
    int a[10];
    int* const & r;

    Y() : r(a) {}
};

Here, a is a named entity (an lvalue), but in the initializer of r, the expression a is a prvalue (namely the result of the decay of the array).

The overall message is that you should be careful with the language feature that allows const lvalue references to bind to rvalues. Its main purpose is to make function calls easy, but its other uses are much hairier.

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