NAME

std::generate_n - std::generate_n

Synopsis

Defined in header <algorithm>
template< class OutputIt, class Size, class Generator > (until C++11)
void generate_n( OutputIt first, Size count, Generator g );
template< class OutputIt, class Size, class Generator > (since C++11)
OutputIt generate_n( OutputIt first, Size count, Generator g (until C++20)
);
template< class OutputIt, class Size, class Generator >
constexpr OutputIt generate_n( OutputIt first, Size count, (since C++20)
Generator g ); (1)
template< class ExecutionPolicy, class ForwardIt , class Size,
class Generator >

ForwardIt generate_n( ExecutionPolicy&& policy, ForwardIt (2) (since C++17)
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. This overload does 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 - the beginning of the range of elements to generate
count - number of the elements to generate
policy - the execution policy to use. See execution policy for details.
generator function object that will be called.

The signature of the function should be equivalent to the following:
g -
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 must meet the requirements of LegacyOutputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

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

Complexity

Exactly count invocations of g() and assignments, for count>0.

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.

Possible implementation

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++ = g();
}
return first;
}

Example

// Run this code

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

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