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) |
template< class ExecutionPolicy, class ForwardIt, class Size, class Generator > |
(2) | (since C++17) |
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.
Contents |
[edit] 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:
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.
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-ForwardIt must meet the requirements of LegacyForwardIterator.
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[edit] Return value
Iterator one past the last element assigned if count > 0, first otherwise.
[edit] Complexity
Exactly std::max(0, count) invocations of g() and assignments.
[edit] 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 otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory, std::bad_alloc is thrown.
[edit] 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; } |
[edit] 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
[edit] 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 |
[edit] 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 (algorithm function object) |