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Difference between revisions of "cpp/memory/construct at"

From cppreference.com
< cpp‎ | memory
(simplify. use less standardese)
(→‎Example: bit_cast of an initialised array produces an initialised object, add destroy_at call to uninitialise it)
 
(13 intermediate revisions by 7 users not shown)
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{{cpp/title|construct_at}}
 
{{cpp/title|construct_at}}
 
{{cpp/memory/navbar}}
 
{{cpp/memory/navbar}}
−
{{dcl begin}}
+
{{ddcl|header=memory|since=c++20|
−
{{dcl header | memory}}
+
template< class T, class... Args >
−
{{dcl | since=c++20 |
+
−
template<class T, class... Args>
+
 
constexpr T* construct_at( T* p, Args&&... args );
 
constexpr T* construct_at( T* p, Args&&... args );
 
}}
 
}}
−
{{dcl end}}
 
  
−
Creates a {{tt|T}} object initialized with arguments {{tt|args...}} at given address {{tt|p}}. {{cpp/enable_if|{{c|::new(std::declval<void*>()) T(std::declval<Args>()...)}} is well-formed in evaluated context}}.
+
Creates a {{tt|T}} object initialized with arguments {{c|args...}} at given address {{c|p}}. Specialization of this function template participates in overload resolution only if {{c|::new(std::declval<void*>()) T(std::declval<Args>()...)}} is well-formed in an unevaluated context.
  
 
Equivalent to
 
Equivalent to
−
{{source|return ::new (const_cast<void*>(static_cast<const volatile void*>(p)))
+
{{source|return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);}}
−
    T(std::forward<Args>(args)...);
+
−
}}
+
 
except that {{tt|construct_at}} may be used in evaluation of [[cpp/language/constant expression|constant expressions]].
 
except that {{tt|construct_at}} may be used in evaluation of [[cpp/language/constant expression|constant expressions]].
  
−
When {{tt|construct_at}} is called in the evaluation of some constant expression {{c|e}}, the argument {{tt|p}} must point to either storaged obtained by {{c|std::allocator<T>::allocate}} or an object whose lifetime began within the evaluation of {{c|e}}.
+
When {{tt|construct_at}} is called in the evaluation of some constant expression {{c|e}}, the argument {{tt|p}} must point to either storage obtained by {{c|std::allocator<T>::allocate}} or an object whose lifetime began within the evaluation of {{c|e}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | p | pointer to the uninitialized storage on which a {{tt|T}} object will be constructed}}
+
{{par|p|pointer to the uninitialized storage on which a {{tt|T}} object will be constructed}}
−
{{par | args... | arguments used for initialization}}
+
{{par|args...|arguments used for initialization}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
−
{{tt|p}}
+
{{c|p}}
  
 
===Example===
 
===Example===
−
{{example}}
+
{{example
 +
|code=
 +
#include <bit>
 +
#include <memory>
 +
 
 +
class S
 +
{
 +
    int x_;
 +
    float y_;
 +
    double z_;
 +
public:
 +
    constexpr S(int x, float y, double z) : x_{x}, y_{y}, z_{z} {}
 +
    [[nodiscard("no side-effects!")]]
 +
    constexpr bool operator==(const S&) const noexcept = default;
 +
};
 +
 
 +
consteval bool test()
 +
{
 +
    alignas(S) unsigned char storage[sizeof(S)]{};
 +
    S uninitialized = std::bit_cast<S>(storage);
 +
    std::destroy_at(&uninitialized);
 +
    S* ptr = std::construct_at(std::addressof(uninitialized), 42, 2.71f, 3.14);
 +
    const bool res{*ptr == S{42, 2.71f, 3.14}<!---->};
 +
    std::destroy_at(ptr);
 +
    return res;
 +
}
 +
static_assert(test());
 +
 
 +
int main() {}
 +
}}
 +
 
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3870|std=C++20|before={{tt|construct_at}} could create objects of a cv-qualified types|after=only cv-unqualified types are permitted}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/memory/allocator_traits/dsc construct}}
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{{dsc inc|cpp/memory/allocator/dsc allocate}}
−
{{dsc inc | cpp/memory/dsc destroy_at}}
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{{dsc inc|cpp/memory/allocator_traits/dsc construct}}
 +
{{dsc inc|cpp/memory/dsc destroy_at}}
 +
{{dsc inc|cpp/memory/ranges/dsc construct_at}}
 
{{dsc end}}
 
{{dsc end}}
  
−
{{langlinks|ja|zh}}
+
{{langlinks|es|ja|ru|zh}}

Latest revision as of 03:58, 3 May 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)



 
Defined in header <memory>
template< class T, class... Args >
constexpr T* construct_at( T* p, Args&&... args );
 (since C++20)

Creates a T object initialized with arguments args... at given address p. Specialization of this function template participates in overload resolution only if ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed in an unevaluated context.

Equivalent to 

return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);

except that construct_at may be used in evaluation of constant expressions.

When construct_at is called in the evaluation of some constant expression e, the argument p must point to either storage obtained by std::allocator<T>::allocate or an object whose lifetime began within the evaluation of e.

Contents

[edit] Parameters

p - pointer to the uninitialized storage on which a T object will be constructed
args... - arguments used for initialization

[edit] Return value

p

[edit] Example

Run this code
#include <bit>
#include <memory>
 
class S
{
    int x_;
    float y_;
    double z_;
public:
    constexpr S(int x, float y, double z) : x_{x}, y_{y}, z_{z} {}
    [[nodiscard("no side-effects!")]]
    constexpr bool operator==(const S&) const noexcept = default;
};
 
consteval bool test()
{
    alignas(S) unsigned char storage[sizeof(S)]{};
    S uninitialized = std::bit_cast<S>(storage);
    std::destroy_at(&uninitialized);
    S* ptr = std::construct_at(std::addressof(uninitialized), 42, 2.71f, 3.14);
    const bool res{*ptr == S{42, 2.71f, 3.14}};
    std::destroy_at(ptr);
    return res;
}
static_assert(test());
 
int main() {}

[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 3870 C++20 construct_at could create objects of a cv-qualified types only cv-unqualified types are permitted

[edit] See also

 allocates uninitialized storage
(public member function of std::allocator<T>) [edit]
[static]
 constructs an object in the allocated storage
(function template) [edit]
(C++17)
 destroys an object at a given address
(function template) [edit]
(C++20)
 creates an object at a given address
(niebloid)[edit]
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