# std::generate_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class OutputIt, class Size, class Generator > void generate_n( OutputIt first, Size count, Generator g ); |  | (until C++11)
template< class OutputIt, class Size, class Generator > OutputIt generate_n( OutputIt first, Size count, Generator g ); |  | (since C++11) (until C++20)
template< class OutputIt, class Size, class Generator > constexpr OutputIt generate_n( OutputIt first, Size count, Generator g ); |  | (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 only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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.

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

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

### 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 >
    OutputIt 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

### 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)

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