std::copy,std::copy_if(3)

NAME
std::copy,std::copy_if - std::copy,std::copy_if

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt >
   OutputIt copy( InputIt first, InputIt last, (1) (constexpr since C++20)
                  OutputIt d_first );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2 >
   ForwardIt2 copy( ExecutionPolicy&& policy, (2) (since C++17)
                    ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 d_first );
   template< class InputIt, class OutputIt, class UnaryPred
   >
                        (3) (since C++11)
   OutputIt copy_if( InputIt first, InputIt last, (constexpr since C++20)
                     OutputIt d_first, UnaryPred pred );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2, class
   UnaryPred >
   ForwardIt2 copy_if( ExecutionPolicy&& policy, (4) (since C++17)
                       ForwardIt1 first, ForwardIt1 last,
                       ForwardIt2 d_first, UnaryPred pred
   );
   Copies the elements in the range, defined by [first, last), to another range
   beginning at d_first (copy destination range).
   1) Copies all elements in the range [first, last) starting from first and proceeding
   to last.
   If d_first is in [first, last), the behavior is undefined. In this case,
   std::copy_backward may be used instead.
   2) Copies the elements, but executed according to policy.
   This overload participates in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)
   If [first, last) and the copy destination range overlaps, the behavior is undefined.
   3) Only copies the elements for which the predicate pred returns true. This copy
   algorithm is stable: the relative order of the elements that are copied is
   preserved.
   If [first, last) and the copy destination range overlaps, the behavior is undefined.
   4) Same as (3), but executed according to policy.
   This overload participates in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)

Parameters
   first, last - the range of elements to copy
   d_first - the beginning of the destination range
   policy - the execution policy to use. See execution policy for details.
                 unary predicate which returns true for the required elements.
                 The expression pred(v) must be convertible to bool for every argument
   pred - v of type (possibly const) VT, where VT is the value type of InputIt,
                 regardless of value category, and must not modify v. Thus, a parameter
                 type of VT&is not allowed
                 , nor is VT unless for VT a move is equivalent to a copy
                 (since C++11).

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   OutputIt must meet the requirements of LegacyOutputIterator.
   -
   ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
   -
   UnaryPred must meet the requirements of Predicate.

Return value
   Output iterator to the element in the destination range, one past the last element
   copied.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1,2) Exactly \(\scriptsize N\)N assignments.
   3,4) Exactly \(\scriptsize N\)N applications of the predicate pred, and at most
   \(\scriptsize N\)N assignments.
   For the overloads with an ExecutionPolicy, there may be a performance cost if
   ForwardIt1's value type is not MoveConstructible.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
                        copy (1)
   template<class InputIt, class OutputIt>
   OutputIt copy(InputIt first, InputIt last,
                 OutputIt d_first)
   {
       for (; first != last; (void)++first, (void)++d_first)
           *d_first = *first;
       return d_first;
   }
                        copy_if (3)
   template<class InputIt, class OutputIt, class UnaryPred>
   OutputIt copy_if(InputIt first, InputIt last,
                    OutputIt d_first, UnaryPred pred)
   {
       for (; first != last; ++first)
           if (pred(*first))
           {
               *d_first = *first;
               ++d_first;
           }
       return d_first;
   }

Notes
   In practice, implementations of std::copy avoid multiple assignments and use bulk
   copy functions such as std::memmove if the value type is TriviallyCopyable and the
   iterator types satisfy LegacyContiguousIterator.
   When copying overlapping ranges, std::copy is appropriate when copying to the left
   (beginning of the destination range is outside the source range) while
   std::copy_backward is appropriate when copying to the right (end of the destination
   range is outside the source range).

Example
   The following code uses std::copy to both copy the contents of one std::vector to
   another and to display the resulting std::vector. // Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <numeric>
 #include <vector>
 int main()
 {
     std::vector<int> from_vector(10);
     std::iota(from_vector.begin(), from_vector.end(), 0);
     std::vector<int> to_vector;
     std::copy(from_vector.begin(), from_vector.end(),
               std::back_inserter(to_vector));
 // or, alternatively,
 // std::vector<int> to_vector(from_vector.size());
 // std::copy(from_vector.begin(), from_vector.end(), to_vector.begin());
 // either way is equivalent to
 // std::vector<int> to_vector = from_vector;
     std::cout << "to_vector contains: ";
     std::copy(to_vector.begin(), to_vector.end(),
               std::ostream_iterator<int>(std::cout, " "));
     std::cout << '\n';
     std::cout << "odd numbers in to_vector are: ";
     std::copy_if(to_vector.begin(), to_vector.end(),
                  std::ostream_iterator<int>(std::cout, " "),
                  [](int x) { return x % 2 != 0; });
     std::cout << '\n';
     std::cout << "to_vector contains these multiples of 3: ";
     to_vector.clear();
     std::copy_if(from_vector.begin(), from_vector.end(),
                  std::back_inserter(to_vector),
                  [](int x) { return x % 3 == 0; });
     for (const int x : to_vector)
         std::cout << x << ' ';
     std::cout << '\n';
 }

Possible output:
 to_vector contains: 0 1 2 3 4 5 6 7 8 9
 odd numbers in to_vector are: 1 3 5 7 9
 to_vector contains these multiples of 3: 0 3 6 9
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2039 C++11 the return value of std::copy_if was not specified
                       specified
   LWG 2044 C++11 the stability of std::copy_if was not defined defined

See also
   copy_backward copies a range of elements in backwards order
                   (function template)
   reverse_copy creates a copy of a range that is reversed
                   (function template)
   copy_n copies a number of elements to a new location
   (C++11) (function template)
   fill copy-assigns the given value to every element in a range
                   (function template)
   remove_copy copies a range of elements omitting those that satisfy specific
   remove_copy_if criteria
                   (function template)
   ranges::copy
   ranges::copy_if copies a range of elements to a new location
   (C++20) (niebloid)
   (C++20)