Where is erase_if?

Question

I\'ve got a container and would like to erase elements based on a predicate. erase_if sounds familiar, but I can\'t find it in C++. What\'s the name and where i

Answer 1

MSVC does not yet implement C++20's std::erase_if from P1209R0 so, as a workaround, you can just copy the implementation for specific container from that paper somewhere to your code. GCC 9.1 and Clang 9.0 already have it.

namespace std {

// for std::string
template <class charT, class traits, class A, class Predicate>
void erase_if(basic_string<charT, traits, A>& c, Predicate pred) {
    c.erase(remove_if(c.begin(), c.end(), pred), c.end());
}

// for std::deque
template <class T, class A, class Predicate>
void erase_if(deque<T, A>& c, Predicate pred) {
    c.erase(remove_if(c.begin(), c.end(), pred), c.end());
}

// for std::vector
template <class T, class A, class Predicate>
void erase_if(vector<T, A>& c, Predicate pred) {
    c.erase(remove_if(c.begin(), c.end(), pred), c.end());
}

// for std::list
template <class T, class A, class Predicate>
void erase_if(list<T, A>& c, Predicate pred) {
    c.remove_if(pred);
}

// for std::forward_list
template <class T, class A, class Predicate>
void erase_if(forward_list<T, A>& c, Predicate pred) {
    c.remove_if(pred);
}

// for std::map
template <class K, class T, class C, class A, class Predicate>
void erase_if(map<K, T, C, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::multimap
template <class K, class T, class C, class A, class Predicate>
void erase_if(multimap<K, T, C, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::set
template <class K, class C, class A, class Predicate>
void erase_if(set<K, C, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::multiset
template <class K, class C, class A, class Predicate>
void erase_if(multiset<K, C, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::unordered_map
template <class K, class T, class H, class P, class A, class Predicate>
void erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::unordered_multimap
template <class K, class T, class H, class P, class A, class Predicate>
void erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::unordered_set
template <class K, class H, class P, class A, class Predicate>
void erase_if(unordered_set<K, H, P, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

// for std::unordered_multiset
template <class K, class H, class P, class A, class Predicate>
void erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred) {
    for (auto i = c.begin(), last = c.end(); i != last; )
        if (pred(*i))
            i = c.erase(i);
        else
            ++i;
}

} // namespace std

Answer 2

You're probably looking for std::remove_if, in a pattern such as:

vec.erase(std::remove_if(vec.begin(), vec.end(), predicate), vec.end());

Answer 3

I'm guessing you're thinking of remove_if which takes a predicate to determine if the element ought to be removed.

remove_if returns an iterator pointing to the beginning of the elements to remove in the container. To actually remove them you need to use erase:

container.erase(remove_if(container.start(), container.end(), pred), container.end())

---

Either that or perhaps you mistakenly recalled the copy_if algorithm? Which somehow got left out of the standard but was written about - and implemented - in Effective STL.

Answer 4

It's in Library Fundamentals v2, and soon in C++20.

Answer 5

Actually there exists a method called erase_if in the Boost library for pointer containers.

Answer 6

There is a list::remove_if, but not for all container classes. remove_if also exists as a algorithm, which can be used with the iterators you can get from begin() and end().