I am trying to efficiently make a copy of a vector. I see two possible approaches:
std::vector<int> copyVecFast1(const std::vector<int>& original)
{
std::vector<int> newVec;
newVec.reserve(original.size());
std::copy(original.begin(), original.end(), std::back_inserter(newVec));
return newVec;
}
std::vector<int> copyVecFast2(std::vector<int>& original)
{
std::vector<int> newVec;
newVec.swap(original);
return newVec;
}
Which of these is preferred, and why? I am looking for the most efficient solution that will avoid unnecessary copying.
They aren't the same though, are they? One is a copy, the other is a swap. Hence the function names.
My favourite is:
a = b;
Where a
and b
are vectors.
This is another valid way to make a copy of a vector, just use its constructor:
std::vector<int> newvector(oldvector);
This is even simpler than using std::copy
to walk the entire vector from start to finish to std::back_insert
them into the new vector.
That being said, your .swap()
one is not a copy, instead it swaps the two vectors. You would modify the original to not contain anything anymore! Which is not a copy.
new_vector.assign(old_vector.begin(),old_vector.end()); // Method 1
new_vector = old_vector; // Method 2
=
operatorWe can use the public member function std::vector::operator=
of the container std::vector
for assigning values from a vector to another.
Besides, a constructor function also makes sense. A constructor function with another vector as parameter(e.g. x
) constructs a container with a copy of each of the elements in x
, in the same order.
std::vector::swap
std::vector::swap
is not copying a vector to another, it is actually swapping elements of two vectors, just as its name suggests. In other words, the source vector to copy from is modified after std::vector::swap
is called, which is probably not what you are expected.
If the elements in the source vector are pointers to other data, then a deep copy is wanted sometimes.
According to wikipedia:
A deep copy, meaning that fields are dereferenced: rather than references to objects being copied, new copy objects are created for any referenced objects, and references to these placed in B.
Actually, there is no currently a built-in way in C++ to do a deep copy. All of the ways mentioned above are shallow. If a deep copy is necessary, you can traverse a vector and make copy of the references manually. Alternatively, an iterator can be considered for traversing. Discussion on iterator is beyond this question.
you should not use swap to copy vectors, it would change the "original" vector.
pass the original as a parameter to the new instead.
Source: Stackoverflow.com