Does an unused member variable take up memory?

Question

Does initializing a member variable and not referencing/using it further take up RAM during runtime, or does the compiler simply ignore that variable?

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Answer 1

It's dependent on your compiler and its optimization level.

In gcc, if you specify -O, it will turn on the following optimization flags:

-fauto-inc-dec 
-fbranch-count-reg 
-fcombine-stack-adjustments 
-fcompare-elim 
-fcprop-registers 
-fdce
-fdefer-pop
...

-fdce stands for Dead Code Elimination.

You can use __attribute__((used)) to prevent gcc eliminating an unused variable with static storage:

This attribute, attached to a variable with static storage, means that the variable must be emitted even if it appears that the variable is not referenced.

When applied to a static data member of a C++ class template, the attribute also means that the member is instantiated if the class itself is instantiated.