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, (until C++20)
   OutputIt d_first );
   template< class InputIt, class OutputIt >
   constexpr OutputIt copy( InputIt first, InputIt (since C++20)
   last, 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
   UnaryPredicate >
                        (since C++11)
   OutputIt copy_if( InputIt first, InputIt last, (until C++20)
   OutputIt d_first,
   UnaryPredicate pred ); (1)
   template< class InputIt, class OutputIt, class
   UnaryPredicate >
   constexpr OutputIt copy_if( InputIt first, (since C++20)
   InputIt last, (3)
   OutputIt d_first,
   UnaryPredicate pred );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2, class
   UnaryPredicate >
   ForwardIt2 copy_if( ExecutionPolicy&& policy, (4) (since C++17)
   ForwardIt1 first, ForwardIt1 last,
   ForwardIt2 d_first,
   UnaryPredicate pred );
   Copies the elements in the range, defined by [first, last), to another range
   beginning at d_first.
   1) Copies all elements in the range [first, last) starting from first and proceeding
   to last - 1. The behavior is undefined if d_first is within the range [first, last).
   In this case, std::copy_backward may be used instead.
   3) Only copies the elements for which the predicate pred returns true. The relative
   order of the elements that are copied is preserved. The behavior is undefined if the
   source and the destination ranges overlap.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

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.
   -
   UnaryPredicate must meet the requirements of Predicate.

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

Complexity
   1-2) Exactly (last - first) assignments
   3-4) Exactly (last - first) applications of the predicate, between 0 and (last
   - first) assignments (assignment for every element for which predicate is equal to
   true, dependent on predicate and input data)
   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.

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).

Possible implementation

First version
   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;
   }

Second version
   template<class InputIt, class OutputIt, class UnaryPredicate>
   OutputIt copy_if(InputIt first, InputIt last,
                    OutputIt d_first, UnaryPredicate pred)
   {
       for (; first != last; ++first) {
           if (pred(*first)) {
               *d_first = *first;
               ++d_first;
           }
       }
       return d_first;
   }

Example
   The following code uses copy to both copy the contents of one vector to another and
   to display the resulting vector: // Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 #include <iterator>
 #include <numeric>
 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:\n";
     to_vector.clear();
     std::copy_if(from_vector.begin(), from_vector.end(),
                  std::back_inserter(to_vector),
                  [](int x) { return x % 3 == 0; });
     for (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

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)