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?

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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).