std::generate_n
Defined in header <algorithm>
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template
<
class OutputIt, class Size, class Generator >
OutputIt generate_n( OutputIt first, Size count, Generator g ) ; |
(1) | (constexpr since C++20) |
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template
<
class ExecutionPolicy,
class ForwardIt, class Size, class Generator >
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(2) | (since C++17) |
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std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true |
(until C++20) |
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std::is_execution_policy_v < std::remove_cvref_t <ExecutionPolicy>> is true |
(since C++20) |
If Size is not convertible to an integral type
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:
The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret |
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| Type requirements | ||||||||
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OutputIt must meet the requirements of LegacyOutputIterator
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ForwardIt must meet the requirements of LegacyForwardIterator
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Return value
Iterator one past the last element assigned if count > 0, first
Complexity
Exactly std::max(0, count) invocations of g()
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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy - 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
#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 |
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
| copy-assigns the given value to N elements in a range (function template) |
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| assigns the results of successive function calls to every element in a range (function template) |
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(C++20)
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saves the result of N applications of a function (algorithm function object) |