how to implement a for_each like function in c++? - c++

Question

I need to implement a for_each function, like below. I know std::for_each could apply fn to each element, but we cannot erase elements in std::for_each. I need to extend this template function, so that in fn, the caller can both visit elements and erase elements one at a time. Is there a proper way to do this?
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
class A
{
public:
explicit A(){
mVec.clear();
}
~A(){}
template<class T> void for_each(T fn)
{
for(size_t i = 0; i < mVec.size(); ++i)
{
//fn can erase element or just visit element
fn(mVec[i]);
}
}
vector<int> mVec;
};
int main()
{
A test;
for(int i = 0; i < 8; ++i)
{
test.mVec.push_back(i);
}
test.for_each([&test](int i){
if (i % 2 == 0)
{
cout << i << " deleted" << endl;
test.mVec.erase(find(test.mVec.begin(), test.mVec.end(), i));
}
else
{
cout << i << " parse" << endl;
}
});
system("pause");
return 0;
}
Edit: In for_each template function, we do not know whether the caller will erase elements or not. Erasing elements is done in fn

Answer 1

Could you return a bool value from the function, where true means "erase the element"? Then your for_each function becomes something like.
size_t i = 0;
for(size_t j = 0; j < mVec.size(); ++j) {
if (!fn(mVec[j])) {
// The element must be kept
if (i != j)
mVec[i] = std::move(mVec[j]);
i++;
}
}
mVec.resize(i);
The advantage is also that this is always O(n), no matter how many elements are erased.
EDIT: The loop above is really just std::remove_if(), so #ChenOT's suggestion is the best. Alternatively
n = std::remove_if(mVec.begin(), mVec.end(), fn) - mVec.begin();
mVec.resize(n);