C++ local variables and registers

Question

I often saw that compiler put local function variables in registers. And I have a question regarding this.

If I use some class member variable (integral/pointer, etc...) heavily, does it make sense, to temporary copy it to local variable, work with it, and than copy result to class member?

For example (fill one way ptr list):

struct MyClass{

    struct ObjectHolder{
        ObjectHolder* next_free;
    };
    ObjectHolder *next_free = nullptr;

    void fill(){
        ObjectHolder *copy_of_free = next_free ;  // copy to register?

        for (int i = 0; i < capacity; ++i) {
           ObjectHolder &obj = array[i];

           // build chain of pointers
           obj.next_free = copy_of_free;
           copy_of_free  = &obj;
        }

        next_free = copy_of_free;                 // back to memory
    }
}

Answer 1

What you described in your code goes beyond copying of member variables into local variables to enable register placement. In essence, your question illustrates the difference between two approaches:

1. Fully computing a result locally before writing it to member variables, vs.
2. Writing intermediate results to member variables as you go through computations.

Your program follows approach #1. It is more robust than #2, because the object remains in a consistent state while the computation is performed.

Approach #1 may give the compiler more opportunities to optimize your code, too. However, this is a secondary effect; your object staying consistent during the computation is more important.