c++ - 为什么我们需要 remove_copy_if？

Question

根据http://www.cplusplus.com中的代码

template <class InputIterator, class OutputIterator, class UnaryPredicate>
  OutputIterator remove_copy_if (InputIterator first, InputIterator last,
                                 OutputIterator result, UnaryPredicate pred)
{
  while (first!=last) {
    if (!pred(*first)) {
      *result = *first;
      ++result;
    }
    ++first;
  }
  return result;
}

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;
}

唯一的区别是

 if (!pred(*first)) //copy if is if (pred(*first)) {

remove_copy_if 可以使用模拟

copy_if(it1, it2, not1(pred));

那么为什么我们需要 remove_copy_if？

Answer 1

为了了解历史，最好的问题是，为什么我们需要 copy_if？

如果你看，remove_copy_if之前是c++11 , 而 copy_if在 c++11 中首次亮相.

你是对的，仿函数结果的反转是将这两者分开的全部，但正数更容易理解，因此引入了 copy_if。引用自JaggedSpire's link :

This is a frequently requested addition, mentioned in (for example) the latest edition of The C++ Programming Language. It is formally redundant, since it's just the inverse of remove_copy_if: copying all elements that satisfy a predicate p is just the same as not copying all elements that satisfy !p. It's worth adding anyway. First, C++ isn't really a functional language and transforming a predicate into its negation is sometimes awkward. Second, the workaround of using double negatives is not unconfusing.