function that may work in-place - c++

Question

I have the following function:
shared_ptr_of_points filter(const shared_ptr_of_points input){
shared_ptr_of_points result = make_new();
for(const auto item:input->points){
if(cond(item)){
result->add_point(item);
}
}
return result;
}
A typical use is:
auto filtered_points = filter(some_noisy_points);
However, it may be used in another way which is:
some_points = filter(some_points);
The above implementation works fine in this case. However, an unnecessary copy is done.
The question: What is the classical solution for this problem?
P.S. I need both cases to work fine where in the first one, the copy should happen (I have to accept the argument as const). In the second case, no copy should happen.
While overloading is OK, making two separated functions is not an option.

Answer 1

You could achieve your goal nicely by using a slightly different prototype:
void filter(const shared_ptr_of_points & input, shared_ptr_of_points & output)
If you are willing to use it, then a possible implementation will look like this:
// Example program
#include <iostream>
#include <string>
#include <memory>
#include <vector>
#include <algorithm>
typedef std::shared_ptr<std::vector<int>> shared_ptr_of_points;
bool cond(int x){
return x > 100;
}
void filterCopy(const shared_ptr_of_points & input, shared_ptr_of_points & output){
if (!output){
output = std::make_shared<std::vector<int>>();
}
for(const auto item:*input){
if(cond(item)){
output->push_back(item);
}
}
}
void filterInplace(shared_ptr_of_points & inout){
inout->erase(std::remove_if(inout->begin(), inout->end(), [](int x){return !cond(x);}), inout->end());
}
void filter(const shared_ptr_of_points & input, shared_ptr_of_points & output){
if (output == input)
filterInplace(output);
else
filterCopy(input, output);
}
int main()
{
shared_ptr_of_points pts = std::make_shared<std::vector<int>>();
pts->emplace_back(100);
pts->emplace_back(200);
pts->emplace_back(300);
for(const auto item:*pts){
std::cout << item << std::endl;
}
std::cout << "**********" << std::endl;
shared_ptr_of_points resCopy;
filter(pts, resCopy);
for(const auto item:*resCopy){
std::cout << item << std::endl;
}
std::cout << "**********" << std::endl;
filter(pts, pts);
for(const auto item:*pts){
std::cout << item << std::endl;
}
}