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std::allocator_traits<Alloc>::construct - cppreference.com

From cppreference.com

template< class T, class... Args >
static void construct( Alloc& a, T* p, Args&&... args );
(since C++11)
(constexpr since C++20)

If possible, constructs an object of type T in allocated uninitialized storage pointed to by p, by calling a.construct(p, std::forward<Args>(args)...).

If the above is not possible (e.g. Alloc does not have the member function construct()), then calls

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

(until C++20)

std::construct_at(p, std::forward<Args>(args)...)

(since C++20)

Parameters

a - allocator to use for construction
p - pointer to the uninitialized storage on which a T object will be constructed
args... - the constructor arguments to pass to a.construct() or to placement-new(until C++20)std::construct_at()(since C++20)

Notes

This function is used by the standard library containers when inserting, copying, or moving elements.

Because this function provides the automatic fall back to placement new, the member function construct() is an optional Allocator requirement.

(since C++11)

Example

#include <memory>

struct foo
{
    virtual int bar() { return 0; }
    virtual ~foo() {}
};

int main()
{
    std::allocator<foo> al;
    using traits = std::allocator_traits<decltype(al)>;
    foo* p = traits::allocate(al, 1);
    traits::construct(al, p);
    p->bar();
    traits::destroy(al, p);
    traits::deallocate(al, p, 1);
}

See also