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

From cppreference.com
< cpp‎ | memory
(Example: ✔)
(→‎Example: bit_cast of an initialised array produces an initialised object, add destroy_at call to uninitialise it)
 
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{{cpp/title|construct_at}}
 
{{cpp/title|construct_at}}
 
{{cpp/memory/navbar}}
 
{{cpp/memory/navbar}}
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{{dcl begin}}
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{{ddcl|header=memory|since=c++20|
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{{dcl header | memory}}
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template< class T, class... Args >
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{{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 );
 
}}
 
}}
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{{dcl end}}
 
  
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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 unevaluated 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
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{{source|return ::new (const_cast<void*>(static_cast<const volatile void*>(p)))
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{{source|return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);}}
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    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]].
  
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===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
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{{par | p | pointer to the uninitialized storage on which a {{tt|T}} object will be constructed}}
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{{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===
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{{tt|p}}
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{{c|p}}
  
 
===Example===
 
===Example===
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{{example|code=
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{{example
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#include <cstdint>
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|code=
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#include <iostream>
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#include <bit>
 
#include <memory>
 
#include <memory>
  
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struct S {
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class S
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     int x;
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{
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     float y;
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     int x_;
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     double z;
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     float y_;
−
 
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     double z_;
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     S(int x, float y, double z) : x{x}, y{y}, z{z} { std::cout << "S::S();\n"; }
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public:
−
      
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     constexpr S(int x, float y, double z) : x_{x}, y_{y}, z_{z} {}
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    ~S() { std::cout << "S::~S();\n"; }
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     [[nodiscard("no side-effects!")]]
−
 
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     constexpr bool operator==(const S&) const noexcept = default;
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     void print() const {
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        std::cout << "S { x=" << x << "; y=" << y << "; z=" << z << "; };\n";
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−
    }
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};
 
};
  
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int main()
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consteval bool test()
 
{
 
{
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     alignas(S) uint8_t storage[sizeof(S)];
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     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());
  
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    S* ptr = std::construct_at(reinterpret_cast<S*>(storage), 42, 2.71828f, 3.1415);
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int main() {}
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    ptr->print();
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}}
  
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    std::destroy_at(reinterpret_cast<S*>(storage));
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===Defect reports===
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}
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{{dr list begin}}
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|output=
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{{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}}
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S::S();
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{{dr list end}}
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S { x=42; y=2.71828; z=3.1415; };
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−
S::~S();
+
−
}}
+
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/memory/allocator/dsc allocate}}
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{{dsc inc|cpp/memory/allocator/dsc allocate}}
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{{dsc inc | cpp/memory/allocator_traits/dsc construct}}
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{{dsc inc|cpp/memory/allocator_traits/dsc construct}}
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{{dsc inc | cpp/memory/dsc destroy_at}}
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{{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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