The task of removing elements with a certain property from a std::vector
or other container lends itself to a functional style implementation: Why bother with loops, memory deallocation and moving data around correctly?
However the standard way of doing this in C++ seems to be the following idiom:
std::vector<int> ints; ... ints.erase( std::remove_if(ints.begin(), ints.end(), [](int x){return x < 0;}), ints.end());
This example removes all elements less than zero from an integer vector.
I find it not only ugly but also easy to use incorrectly. It is clear that std::remove_if
cannot change the size of the vector (as its name would suggest) because it only gets iterators passed. But many developers, including myself, don't get that in the beginning.
So is there a safer and hopefully more elegant way to achieve this? If not, why?
If you need to remove multiple elements from the vector, the std::remove will copy each, not removed element only once to its final location, while the vector::erase approach would move all of the elements from the position to the end multiple times.
vector::erase() erase() function is used to remove elements from a container from the specified position or range.
The C++ vector has many member functions. Two of these member functions are erase() and pop_back(). pop_back() removes the last element from the vector. In order to remove all the elements from the vector, using pop_back(), the pop_back() function has to be repeated the number of times there are elements.
The idea is to swap the last element and an element you want to remove and then pop_back() . If you need to remove the last elemtent - just pop_back() . The order of the vector will not be the same but you get a fast remove method.
I find it not only ugly but also easy to use incorrectly.
Don't worry, we all did at the start.
It is clear that std::remove_if cannot change the size of the vector (as its name would suggest) because it only gets iterators passed. But many developers, including myself, don't get that in the beginning.
Same. It confuses everyone. It probably shouldn't have been called remove_if
all those years ago. Hindsight, eh?
So is there a safer and hopefully more elegant way to achieve this?
No
If not, why?
Because this is the safest, most elegant way that preserves performance when deleting items from a container in which deleting an item invalidates iterators.
anticipating:
Anything I can do?
Yes, wrap this idiom into a function
template<class Container, class F> auto erase_where(Container& c, F&& f) { return c.erase(std::remove_if(c.begin(), c.end(), std::forward<F>(f)), c.end()); }
The call in the motivating example then becomes:
auto is_negative = [](int x){return x < 0;}; erase_where(ints, is_negative);
or
erase_where(ints, [](int x){return x < 0;});
This will become available in a C++17-ready compiler soon through the std::experimental::erase_if
algorithm:
#include <algorithm> #include <iostream> #include <iterator> #include <vector> #include <experimental/vector> int main() { std::vector<int> ints { -1, 0, 1 }; std::experimental::erase_if(ints, [](int x){ return x < 0; }); std::copy(ints.begin(), ints.end(), std::ostream_iterator<int>(std::cout, ",")); }
Live Example that prints 0,1
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