Performance of vector::size() : is it as fast as reading a variable?

Interesting question.

So, what's going to happened ? Well if you debug with gdb you'll see something like 3 member variables (names are not accurate):

• _M_begin: pointer to the first element of the dynamic array
• _M_end: pointer one past the last element of the dynamic array
• _M_capacity: pointer one past the last element that could be stored in the dynamic array

The implementation of vector<T,Alloc>::size() is thus usually reduced to:

return _M_end - _M_begin;  // Note: _Mylast - _Myfirst in VC 2008

Now, there are 2 things to consider when regarding the actual optimizations possible:

• will this function be inlined ? Probably: I am no compiler writer, but it's a good bet since the overhead of a function call would dwarf the actual time here and since it's templated we have all the code available in the translation unit
• will the result be cached (ie sort of having an unnamed local variable): it could well be, but you won't know unless you disassemble the generated code

In other words:

• If you store the size yourself, there is a good chance it will be as fast as the compiler could get it.
• If you do not, it will depend on whether the compiler can establish that nothing else is modifying the vector; if not, it cannot cache the variable, and will need to perform memory reads (L1) every time.

It's a micro-optimization. In general, it will be unnoticeable, either because the performance does not matter or because the compiler will perform it regardless. In a critical loop where the compiler does not apply the optimization, it can be a significant improvement.

As I understand the 1998 C++ specification, vector<T>::size() takes constant time, not linear time. So, this question likely boils down to whether it's faster to read a local variable than calling a function that does very little work.

I'd therefore claim that storing your vector's size() in a local variable will speed up your program by a small amount, since you'll only call that function (and therefore the small constant amount of time it takes to execute) once instead of many times.

Performance of vector::size() : is it as fast as reading a variable?

Probably not.

Does it matter

Probably not.

Unless the work you're doing per iteration is tiny (like one or two integer operations) the overhead will be insignificant.