# std::generate_n

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< cpp‎ | algorithm

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:

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)

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