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Defined in header <memory> | ||
Call signature | ||
template<no-throw-forward-iterator I> requiresstd::default_initializable<std::iter_value_t<I>> | (since C++20) (constexpr since C++26) | |
Constructs objects of typestd::iter_value_t<I> in the uninitialized memory areafirst + [0, count) byvalue-initialization, as if byreturnranges::uninitialized_value_construct(std::counted_iterator(first, count),
std::default_sentinel).base();
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:
Contents |
| first | - | the beginning of the range of elements to initialize |
| count | - | the number of elements to construct |
As described above.
Linear incount.
Any exception thrown on construction of the elements in the destination range.
An implementation may improve the efficiency of theranges::uninitialized_value_construct_n, e.g. by usingranges::fill_n, if the value type of the range is aCopyAssignableTrivialType.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_raw_memory_algorithms | 202411L | (C++26) | constexpr forspecialized memory algorithms |
struct uninitialized_value_construct_n_fn{template<no-throw-forward-iterator I> requiresstd::default_initializable<std::iter_value_t<I>>constexpr I operator()(I first,std::iter_difference_t<I> count)const{auto iter=std::counted_iterator(first, count);returnranges::uninitialized_value_construct(iter,std::default_sentinel).base();}}; inlineconstexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{}; |
#include <iostream>#include <memory>#include <string> int main(){struct S{std::string m{"█▓▒░ █▓▒░ █▓▒░ "};}; constexprint n{4}; alignas(alignof(S))char out[n* sizeof(S)]; try{auto first{reinterpret_cast<S*>(out)};auto last= std::ranges::uninitialized_value_construct_n(first, n); auto count{1};for(auto it{first}; it!= last;++it)std::cout<< count++<<' '<< it->m<<'\n'; std::ranges::destroy(first, last);}catch(...){std::cout<<"Exception!\n";} // For scalar types, uninitialized_value_construct_n// zero-initializes the given uninitialized memory area.int v[]{1,2,3,4,5,6,7,8};std::cout<<' ';for(constint i: v)std::cout<< i<<' ';std::cout<<"\n "; std::ranges::uninitialized_value_construct_n(std::begin(v),std::size(v));for(constint i: v)std::cout<< i<<' ';std::cout<<'\n';}
Output:
1 █▓▒░ █▓▒░ █▓▒░ 2 █▓▒░ █▓▒░ █▓▒░ 3 █▓▒░ █▓▒░ █▓▒░ 4 █▓▒░ █▓▒░ █▓▒░1 2 3 4 5 6 7 80 0 0 0 0 0 0 0
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3870 | C++20 | this algorithm might create objects on aconst storage | kept disallowed |
| constructs objects byvalue-initialization in an uninitialized area of memory, defined by a range (algorithm function object)[edit] | |
| constructs objects bydefault-initialization in an uninitialized area of memory, defined by a range (algorithm function object)[edit] | |
| constructs objects bydefault-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object)[edit] | |
| constructs objects byvalue-initialization in an uninitialized area of memory, defined by a start and a count (function template)[edit] |
