Casting related invalid lvalue in unary &

Question

I'm getting the invalid lvalue error using gcc 3.4.6. The line that invokes the error has a function that receives a pointer to SetElement (which is typedef void*). I'm trying to achieve this using casting from another type (also typedef void*) the following way: &((SetElement)my_var). gcc complains about this. Any idea what can be done?

Answer 1

You are taking the address of a temporary (an rvalue), which cannot be done. This is like trying to do:

&(2+2)
// or
&4
// or
&(my_ptr + 4)

You can create a temporary yourself (thus an lvalue), using one of two methods:

AnotherType **my_var_ptr = &my_var;
SetElement **set_element_ptr = (SetElement *)my_var_ptr;

// or

SetElement *set_element = (SetElement)my_var;
SetElement **set_element_ptr = &set_element;

Or you can simply cast in a different way (solution):

SetElement **set_element_ptr = (SetElement *)&my_var;

This works because you're taking the address of my_var (an lvalue), and not of (SetElement)my_var (an rvalue).