Is there a way to construct a vector
as the concatenation of 2 vector
s (Other than creating a helper function?)
For example:
I came across this question looking for the same thing, and hoping there was an easier way than the one I came up with... seems like there isn't.
So, some iterator trickery should do it if you don't mind a helper template class:
#include <vector>
#include <iostream>
template<class T>
class concat
{
public:
using value_type = typename std::vector<T>::const_iterator::value_type;
using difference_type = typename std::vector<T>::const_iterator::difference_type;
using reference = typename std::vector<T>::const_iterator::reference;
using pointer = typename std::vector<T>::const_iterator::pointer;
using iterator_category = std::forward_iterator_tag;
concat(
const std::vector<T>& first,
const std::vector<T>& last,
const typename std::vector<T>::const_iterator& iterator) :
mFirst{first},
mLast{last},
mIterator{iterator}{}
bool operator!= ( const concat& i ) const
{
return mIterator != i.mIterator;
}
concat& operator++()
{
++mIterator;
if(mIterator==mFirst.end())
{
mIterator = mLast.begin();
}
return *this;
}
reference operator*() const
{
return *mIterator;
}
private:
const std::vector<T>& mFirst;
const std::vector<T>& mLast;
typename std::vector<T>::const_iterator mIterator;
};
int main()
{
const std::vector<int> first{0,1,2,3,4};
const std::vector<int> last{5,6,7,8,9};
const std::vector<int> concatenated(
concat<int>(first,last,first.begin()),
concat<int>(first,last,last.end()));
for(auto i: concatenated)
{
std::cout << i << std::endl;
}
return 0;
}
You may have to implement operator++(int) or operator== depending on how your STL implements the InputIterator constructor, this is the minimal iterator code example I could come up with for MingW GCC.
Have Fun! :)
I think you have to write a help function. I'd write it as:
std::vector<int> concatenate(const std::vector<int>& lhs, const std::vector<int>& rhs)
{
auto result = lhs;
std::copy( rhs.begin(), rhs.end(), std::back_inserter(result) );
return result;
}
The call it as:
const auto concatenation = concatenate(first, second);
If the vectors are likely to be very large (or contain elements that are expensive to copy), then you might need to do a reserve
first to save reallocations:
std::vector<int> concatenate(const std::vector<int>& lhs, const std::vector<int>& rhs)
{
std::vector<int> result;
result.reserve( lhs.size() + rhs.size() );
std::copy( lhs.begin(), lhs.end(), std::back_inserter(result) );
std::copy( rhs.begin(), rhs.end(), std::back_inserter(result) );
return result;
}
(Personally, I would only bother if there was evidence it was a bottleneck).
No, it's not possible if you require that
const
.class Vector : public vector<int>
{
public:
Vector operator+(const Vector& vec);
};
Vector Vector::operator+(const Vector& vec)
{
for (int i = 0; i < vec.size(); i++)
{
this->push_back(vec[i]);
}
return *this;
}
Let me preface this by saying this is a hack, and will not give an answer to how to do this using a vector
. Instead we'll depend on sizeof(int) == sizeof(char32_t)
and use a u32string
to contain our data.
This answer makes it exceedingly clear that only primitives can be used in a basic_string
, and that any primitive larger than 32-bits would require writing a custom char_traits
, but for an int
we can just use u32string
.
The qualification for this can be validated by doing:
static_assert(sizeof(int) == sizeof(char32_t));
Once size equality has been established, and with the knowledge that things like non-const
data
, and emplace
or emplace_back
cannot be used, u32string
can be used like a vector<int>
, with the notable inclusion of an addition opperator:
const vector<int> first = {13};
const vector<int> second = {42};
const u32string concatenation = u32string(first.cbegin(), first.cend()) + u32string(second.cbegin(), second.cend());
[Live Example]
template<typename T>
std::vector<T> operator+(const std::vector<T>& v1, const std::vector<T>& v2){
std::vector<T> vr(std::begin(v1), std::end(v1));
vr.insert(std::end(vr), std::begin(v2), std::end(v2));
return vr;
}
This does require a helper "function", but at least it allows you to use it as
const vector<int> concatenation = first + second;