std::generate_n

Defined in header `<algorithm>`
	(1)
template< class OutputIt, class Size, class Generator > OutputIt generate_n( OutputIt first, Size count, Generator g );			(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>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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 (niebloid)