< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
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

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.


[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. 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 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';


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 “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

[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