std::uninitialized_value_construct

 

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Defined in header <memory>
template<class NoThrowForwardIt > void uninitialized_value_construct( NoThrowForwardIt first,                                     NoThrowForwardIt last );	(1)	(since C++17) (constexpr since C++26)
template<class ExecutionPolicy, class NoThrowForwardIt > void uninitialized_value_construct( ExecutionPolicy&& policy,                                     NoThrowForwardIt first,                                     NoThrowForwardIt last );	(2)	(since C++17)

1) Constructs objects of type typenamestd::iterator_traits<NoThrowForwardIt>::value_type in the uninitialized memory area [first, last) by value-initialization, as if by

for(; first != last;++first)
    ::new(voidify(*first))
        typenamestd::iterator_traits<NoThrowForwardIt>::value_type();

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

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, last	-	the pair of iterators defining the range of elements to initialize
policy	-	the execution policy to use
Type requirements
- NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.

Complexity

Linear in the distance between first and last.

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.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_raw_memory_algorithms	202411L	(C++26)	constexpr for specialized memory algorithms, (1)

Possible implementation

template<class NoThrowForwardIt>constexprvoid uninitialized_value_construct(NoThrowForwardIt first, NoThrowForwardIt last){using Value =typenamestd::iterator_traits<NoThrowForwardIt>::value_type; NoThrowForwardIt current = first;try{for(; current != last;++current){::new(static_cast<void*>(std::addressof(*current))) Value();}}catch(...){std::destroy(first, current);throw;}}

Example

Run this code

#include <iostream>#include <memory>#include <string>   int main(){struct S {std::string m{"Default value"};};   constexprint n{3}; alignas(alignof(S))unsignedchar mem[n * sizeof(S)];   try{auto first{reinterpret_cast<S*>(mem)};auto last{first + n};   std::uninitialized_value_construct(first, last);   for(auto it{first}; it != last;++it)std::cout<< it->m <<'\n';   std::destroy(first, last);}catch(...){std::cout<<"Exception!\n";}   // For scalar types, uninitialized_value_construct// zero-fills the given uninitialized memory area.int v[]{1, 2, 3, 4};for(constint i : v)std::cout<< i <<' ';std::cout<<'\n'; std::uninitialized_value_construct(std::begin(v), std::end(v));for(constint i : v)std::cout<< i <<' ';std::cout<<'\n';}

Output:

Default value Default value Default value 1 2 3 4 0 0 0 0

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 3870	C++20	this algorithm might create objects on a const storage	kept disallowed

See also

uninitialized_value_construct_n (C++17)	constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_default_construct (C++17)	constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template)
ranges::uninitialized_value_construct (C++20)	constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object)