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std::generate_n

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 does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) 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 >
constexpr // since C++20
OutputIt  // void until C++11
generate_n( OutputIt first, Size count, Generator g )
{
    for( Size i = 0; i < count; ++i ) {
        *first++ = g();
    }
    return first;
}

Example

#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

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

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