在空的 std::vector 上调用 resize(0) 是否合法？

Question

I have the following code:

std::vector<int> v; // v has no elements
int size = getSomeNumber(); // this function may return any number that is >= 0
v.resize(size);

As you can see, getSomeNumber may return any positivie number or 0.

Instead of checking if size > 0 before calling resize , I would rather simply call it immediately. And in case size is 0, nothing would happen.

Is this legal code?

Answer 1

Indeed it is legal to call resize(0) on an empty std::vector .

Although std::vector::clear does the same thing, and is clearer.

Note that you can regard the behaviour of v.resize(v.size()) to be a no operation. The C++ standard states that the complexity has to be linear (nb not necessarily proportional ) in the difference between the new size and the current size in the cases where the operation doesn't decrease the number of elements.

Answer 2

It's legal, as @Bathsheba said it has the same effect of calling the clear() method. Also, it is guaranteed no exception is thrown: "If n is less than or equal to the size of the container, the function never throws exceptions (no-throw guarantee)." ( reference ).