Checking whether a vector is empty

Question

Suppose I have a std::vector say Vector

Now after performing some operations on the vector(either insertion or deletion) I want to check if the vector is empty and on the basis of that I want to perform some operations.

Which approach is better

Approach 1

if (Vector.size() == 0){ /* operations */ } 

Approach 2

if (Vector.empty()) { /* operations */ } 

Which is a better approach, 1 or 2?

Answer 1

v.size() == 0 says "I'm comparing the size", but does so to check whether the container empty. There's a small algorithm to digest (very small, as it only consists of a comparison) before you know what it does.
OTOH, v.empty() does exactly what it says: it checks whether v is empty.
Due to this, I clearly prefer #2, as it does what it says. That's why empty() was invented, after all.

But there's also an algorithmic reason to prefer empty(): If someone later changes std::vector into a std::list, v.size() might have O(n). (In C++ 03 it's guaranteed to be O(1) for std::vector, but not for std::list. According to James' comment to Prasoon's answer it will be O(1) for all containers in C++1x.)