std::generate_n() algorithm

since C++20

// (1)
template< class OutputIt, class Size, class Generator >
constexpr OutputIt generate_n( OutputIt first, Size count, Generator g );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Size, class Generator >
ForwardIt generate_n( ExecutionPolicy&& policy, ForwardIt first,
                      Size count, Generator g );

since C++17

// (1)
template< class OutputIt, class Size, class Generator >
OutputIt generate_n( OutputIt first, Size count, Generator g );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Size, class Generator >
ForwardIt generate_n( ExecutionPolicy&& policy, ForwardIt first,
                      Size count, Generator g );

since C++11

// (1)
template< class OutputIt, class Size, class Generator >
OutputIt generate_n( OutputIt first, Size count, Generator g );

until C++11

// (1)
template< class OutputIt, class Size, class Generator >
void generate_n( OutputIt first, Size count, Generator g );

• (1) Assigns values, generated by given function object g, to the first count elements in the range beginning at first, if count > 0.
  Does nothing otherwise.

• (2) Same as (1), but executed according to policy.

Overload Resolution

These overloads participate in overload resolution only if 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	The beginning of the range of elements to generate.
policy	Number of elements to generate.
policy	The execution policy to use. See execution policy for details.
g	Generator function object that will be called. The signature of the function should be equivalent to the following: Ret fun(); The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret.

Type requirements

OutputIt	LegacyOutputIterator
ForwardIt	LegacyForwardIterator

Return value

Iterator one past the last element assigned if count > 0, first otherwise. (since C++11)
(none) (until C++11)

Complexity

Exactly std::max(0, count) invocations of g() and assignments.

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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

generate_n (1)

template<class OutputIt, class Size, class Generator>
constexpr // since C++20
OutputIt  // void until C++11
generate_n(OutputIt first, Size count, Generator g)
{
    for (Size i = 0; i < count; ++i, ++first)
        *first = g();

    return first;
}

Examples

Main.cpp

#include <algorithm>
#include <functional>
#include <iostream>
#include <iterator>
#include <random>

int main()
{
    std::mt19937 rng; // default constructed, seeded with fixed seed
    std::generate_n(std::ostream_iterator<std::mt19937::result_type>(std::cout, " "),
                    5, std::ref(rng));
    std::cout << '\n';
}

Output

3499211612 581869302 3890346734 3586334585 545404204