Is it more efficient to copy a vector by reserving and copying, or by creating and swapping?
C++AlgorithmVectorStlC++ Problem Overview
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.
C++ Solutions
Solution 1 - C++
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.
Solution 2 - C++
Your second example does not work if you send the argument by reference. Did you mean
void copyVecFast(vec<int> original) // no reference
{
vector<int> new_;
new_.swap(original);
}
That would work, but an easier way is
vector<int> new_(original);
Solution 3 - C++
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.
Solution 4 - C++
Direct answer:
- Use a
=
operator
We can use the public member function std::vector::operator=
of the container std::vector
for assigning values from a vector to another.
- Use a constructor function
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.
Caution:
- Do not use
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.
- Deep or shallow copy?
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.
References
Solution 5 - C++
new_vector.assign(old_vector.begin(),old_vector.end()); // Method 1
new_vector = old_vector; // Method 2
Solution 6 - C++
you should not use swap to copy vectors, it would change the "original" vector.
pass the original as a parameter to the new instead.