std::vector<int> a;
std::vector<int> b;
std::vector<int> c;
I would like to concatenate these three vectors by appending b
's and c
's elements to a
. Which is the best way to do this, and why?
1) By using vector::insert
:
a.reserve(a.size() + b.size() + c.size());
a.insert(a.end(), b.begin(), b.end());
a.insert(a.end(), c.begin(), c.end());
b.clear();
c.clear();
2) By using std::copy
:
a.reserve(a.size() + b.size() + c.size());
std::copy(b.begin(), b.end(), std::inserter(a, a.end()));
std::copy(c.begin(), c.end(), std::inserter(a, a.end()));
b.clear();
c.clear();
3) By using std::move
(from C++11
):
a.reserve(a.size() + b.size() + c.size());
std::move(b.begin(), b.end(), std::inserter(a, a.end()));
std::move(c.begin(), c.end(), std::inserter(a, a.end()));
b.clear();
c.clear();
In my opinion, your first solution is the best way to go.
vector<>::insert
is designed to add element so it's the most adequate solution.
You could call reserve
on the destination vector to reserve some space, but unless you add a lot of vector together, it's likely that it wont provide much benefits: vector<>::insert
know how many elements will be added, you will avoid only one reserve
call.
Note: If those were vector
of more complex type (ie a custom class, or even std::string
), then using std::move
could provide you with a nice performance boost, because it would avoid the copy-constructor. For a vector of int
however, it won't give you any benefits.
Note 2: It's worth mentioning that using std::move
will cause your source vector
's content to be unusable.