Pointer vs Variable speed in C++

Question

At a job interview I was asked the question "In C++ how do you access a variable faster, though the normal variable identifier or though a pointer". I must say I did not have a good technical answer to the question so I took a wild guess.

I said that access time will probably be that same as normal variable/identifier is a pointer to the memory address where the value is stored, just like a pointer. In other words, that in terms of speed they both have the same performance, and that pointers are only different because we can specify the memory address we want them to point to.

The interviewer did not seem very convinced/satisfied with my answer (although he did not say anything, just carried on asking something else), therefore I though to come and ask SO'ers wether my answer was accurate, and if not why (from a theory and technical POV).

Answer 1

When you access a "variable", you look up the address, and then fetch the value.

Remember - a pointer IS a variable. So actually, you:

a) look up the address (of the pointer variable),

b) fetch the value (the address stored at that variable)

... and then ...

c) fetch the value at the address pointed to.

So yes, accessing via "pointer" (rather than directly) DOES involve (a bit) of extra work and (slightly) longer time.

Exactly the same thing occurs whether or not it's a pointer variable (C or C++) or a reference variable (C++ only).

But the difference is enormously small.

Answer 2

A variable does not have to live in main memory. Depending on the circumstances, the compiler can store it in a register for all or part of its life, and accessing a register is much faster than accessing RAM.