std::ranges:: construct_at

Creates a T object initialized with arguments args... at given address p . construct_at participates in overload resolution only if :: new ( std:: declval < void * > ( ) ) T ( std:: declval < Args > ( ) ... ) is well-formed in 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 .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup .
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

Return value

p

Possible implementation

struct construct_at_fn
{
    template<class T, class...Args>
        requires
            requires (void* vp, Args&&... args)
                { ::new (vp) T(static_cast<Args&&>(args)...); }
    constexpr T* operator()(T* p, Args&&... args) const
    {
        return std::construct_at(p, static_cast<Args&&>(args)...);
    }
};
 
inline constexpr construct_at_fn construct_at{};

Notes

std::ranges::construct_at behaves exactly same as std::construct_at , except that it is invisible to argument-dependent lookup.

Example

#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) unsigned char 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);
}

S::S();
S { x=42; y=2.71828; z=3.1415; };
S::~S();

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also