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Defined in header <memory> | ||
Call signature | ||
template<class T,class...Args> constexpr T* construct_at( T* location, Args&&...args); | (since C++20) | |
Creates aT object initialized with the arguments inargs at given addresslocation.
Equivalent toifconstexpr(std::is_array_v<T>) return::new(voidify (*location)) T[1]();
else return::new(voidify (*location)) T(std::forward<Args>(args)...);, except thatconstruct_at may be used in evaluation ofconstant expressions(until C++26).
Whenconstruct_at is called in the evaluation of some constant expressionexpr,location must point to either a storage obtained bystd::allocator<T>::allocate or an object whose lifetime began within the evaluation ofexpr.
This overload participates in overload resolution only if all following conditions are satisfied:
Ifstd::is_array_v<T> istrue andsizeof...(Args) is nonzero, the program is ill-formed.
The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:
Contents |
| location | - | pointer to the uninitialized storage on which aT object will be constructed |
| args... | - | arguments used for initialization |
location
std::ranges::construct_at behaves exactly same asstd::construct_at, except that it is invisible to argument-dependent lookup.
#include <iostream>#include <memory> struct S{int x;float y;double z; S(int x,float y,double z): x{x}, y{y}, z{z}{std::cout<<"S::S();\n";} ~S(){std::cout<<"S::~S();\n";} void print()const{std::cout<<"S { x="<< x<<"; y="<< y<<"; z="<< z<<"; };\n";}}; int main(){ alignas(S)unsignedchar buf[sizeof(S)]; S* ptr= std::ranges::construct_at(reinterpret_cast<S*>(buf),42,2.71828f,3.1415); ptr->print(); std::ranges::destroy_at(ptr);}
Output:
S::S();S { x=42; y=2.71828; z=3.1415; };S::~S();The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3436 | C++20 | construct_at could not create objects of array types | can value-initialize bounded arrays |
| LWG 3870 | C++20 | construct_at could create objects of cv-qualified types | only cv-unqualified types are permitted |
(C++20) | destroys an object at a given address (algorithm function object)[edit] |
(C++20) | creates an object at a given address (function template)[edit] |
