std::generate_n

 

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Defined in header <algorithm>
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:

See More 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
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 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
LWG 426	C++98	the complexity requirement was “exactly count invocations or assignments”, 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

See also

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