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Showing content from https://en.cppreference.com/w/cpp/language/../header/../iterator/../algorithm/generate_n.html below:

std::generate_n - cppreference.com

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

(1) (constexpr since C++20) template< class ExecutionPolicy, class ForwardIt, class Size, class Generator > ForwardIt generate_n( ExecutionPolicy&& policy, ForwardIt first, Size count, Generator g ); (2) (since C++17)

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.

This overload participates in overload resolution only if all following conditions are satisfied:

If Size is not convertible to an integral type, the program is ill-formed.

[edit] Parameters first - the beginning of the range of elements to generate count - number of the elements to generate policy - the execution policy to use g - generator function object that will be called.

The signature of the function should be equivalent to the following:

The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret. â

Type requirements -OutputIt must meet the requirements of LegacyOutputIterator. -ForwardIt must meet the requirements of LegacyForwardIterator. [edit] Return value

Iterator one past the last element assigned if count > 0, first otherwise.

[edit] Complexity

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

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports 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.

[edit] Possible implementation

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

[edit] Example

#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

[edit] 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 426 C++98 the complexity requirement was âexactly count invocations
or assignmentsâ, which is broken if count is negative no invocation or assignment
if count is non-positive LWG 865 C++98 the location of the first element following
the generation range was not returned returned [edit] See also copy-assigns the given value to N elements in a range
(function template) [edit] assigns the results of successive function calls to every element in a range
(function template) [edit] saves the result of N applications of a function
(algorithm function object)[edit]

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