using vector::erase for the whole range

Question

Is

v.erase(v.begin(), v.end());

Just as fast as

v.clear();

?

I don't care about little overheads such as extra function calls etc, the compiler will inline that stuff.

The reason I ask is because I have code like the following:

v.erase(v.begin(), last_it);

last_it will usually be the end iterator, but not always. I know that erasing a vector not from the end has a cost, because later elements in the vector will need to be copied down. In the case where last_it is not the end iterator (rare), I can live with that. But I don't want to introduce such overhead when I basically want to clear the vector. So I considered writing my code like this:

if (last_it == v.end())
{
    v.clear();
}
else
{
    v.erase(v.begin(), last_it);
}

I would like to know whether this is necessary to avoid a performance penalty for erasing the whole vector. I would prefer to keep my code clear and use a single-line statement if there is no penalty.

Answer 1

Don't worry about this and write the obvious code using erase. If you erase to the end there won't be any elements left to shift down so there should be minimal performance difference between the two, if any.

Answer 2

Looking into the sourcecode of vector (refering to visual studio 2012 (code snippet below) or SGI (line 434) one), clear is defined as:

void clear() _NOEXCEPT
{   // erase all elements
    erase(begin(), end());
}

So i guess yes.

So in your case, I wouldn't use the if statement, and just do:

v.erase(v.begin(), last_it);