Validate the return value of copy_if - c++

Question

I would like to check if the return value of std::copy_if is valid.
Something like this
auto it=std::copy_if(s.begin(),s.end(),d.begin(),[&](...){...});
if([it]) // ????
{
// do something
}

Answer 1

The return value of copy_if marks the "one past the end" of the destination range. If you passed a range to copy_if that can hold all of your to-be-copied values then you can dereference everything between the begin of your output range up to it - 1.
#include <vector>
#include <algorithm>
#include <iostream>
int main()
{
std::vector<int> s{1,2,3,4,5,6,7,8,9,10};
std::vector<int> d(10);
auto ce = std::copy_if(s.begin(),s.end(),d.begin(),[&](int x){ return x > 5; });
for(auto i = d.begin(); i != ce; ++i)
{
std::cout << *i << "\n";
}
}

Answer 2

A variant of #Pixelchemist's answer is to erase from the returned iterator to the end of the destination. This leaves behind only those elements that satisfied the condition.
#include <vector>
#include <algorithm>
#include <iostream>
int main()
{
std::vector<int> s{1,2,3,4,5,6,7,8,9,10};
std::vector<int> d(10);
auto it = std::copy_if(s.begin(),s.end(),d.begin(),[&](int x){ return x > 5; });
d.erase(it, d.end());
for(int i : d)
{
std::cout << i << "\n";
}
}

Answer 3

copy_if returns a pointer one past the element it last copied. So in your case the destination range would be [d.begin(), it).
Not sure what type of error checking you would want to do but you can for example use the pointers to see how many elements were copied:
unsigned int elementsCopied = it - d.begin();
You can also iterate over the elements copied:
for(auto i = d.begin(); i != it; ++i)
{
//perform action on i
}
If you are still unsure about how copy_if works, c++ reference gives a clear explanation in my opinion.
Perhaps also interesting, this is a possible implementation of copy_if so you can see what is going on:
template <class InputIterator, class OutputIterator, class UnaryPredicate>
OutputIterator copy_if (InputIterator first, InputIterator last,
OutputIterator result, UnaryPredicate pred)
{
while (first!=last) {
if (pred(*first)) {
*result = *first;
++result;
}
++first;
}
return result;
}