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std::generate_n(3) C++ Standard Libary std::generate_n(3) NAME std::generate_n - std::generate_n Synopsis Defined in header <algorithm> template< class OutputIt, class Size, class Generator > OutputIt generate_n( OutputIt first, Size count, (1) (const- expr since C++20) Generator g ); template< class ExecutionPolicy, class ForwardIt, class Size, class Generator > ForwardIt generate_n( ExecutionPolicy&& policy, (2) (since C++17) ForwardIt first, Size count, Generator g ); 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 other- wise. 2) Same as (1), but executed according to policy. This overload participates in overload resolution only if 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. 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 de- tails. generator function object that will be called. The signature of the function should be equivalent to the following: g - 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 oth- erwise. 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::termi- nate 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::re- sult_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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior the complexity requirement was exactly no invoca- tion or LWG 426 C++98 count invocations assign- ment or assignments, which is broken if if count is non-positive count is negative the location of the first element LWG 865 C++98 following returned the generation range was not returned See also fill_n copy-assigns the given value to N elements in a range (function template) assigns the results of successive function calls to every element generate in a range (function template) ranges::generate_n saves the result of N applications of a function (C++20) (niebloid) http://cppreference.com 2024.06.10 std::generate_n(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Example | Output: | See also
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