NAME¶

std::generate_n - std::generate_n

Synopsis¶

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

class ForwardIt, class Size, class Generator >
ForwardIt generate_n( ExecutionPolicy&& policy, (2) (since C++17)

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

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¶

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

Complexity¶

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

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 generate_n(OutputIt first, Size count, Generator g)
{
for (Size i = 0; i < count; ++i, ++first)
*first = g();

return first;
}

Example¶

// Run this code

#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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the complexity requirement was “exactly no invocation or
LWG 426 C++98 count invocations assignment
or assignments”, which is broken if if count is non-positive
count is negative
the location of the first element
LWG 865 C++98 following returned
the generation range was not returned

See also¶

fill_n copy-assigns the given value to N elements in a range
(function template)
assigns the results of successive function calls to every element
generate in a range
(function template)
ranges::generate_n saves the result of N applications of a function
(C++20) (niebloid)