std::allocator<T>::deallocate

Since C++20

constexpr void deallocate( T* p, std::size_t n );

Until C++20

void deallocate( T* p, std::size_t n );

Deallocates the storage referenced by the pointer p, which must be a pointer obtained by an earlier call to allocate() or allocate_at_least()  (since C++23).

The argument n must be equal to the first argument of the call to allocate() that originally produced p,
or in the range [m, count] if p is obtained from a call to allocate_at_least(m) which returned {p, count}  (since C++23)
otherwise, the behavior is undefined.

Calls ::operator delete(void*) or ::operator delete(void*, std::align_val_t)  (since C++17), but it is unspecified when and how it is called .
In evaluation of a constant expression, this function must deallocate storage allocated within the evaluation of the same expression.  (since C++20)

Parameters

p - pointer obtained from allocate() or allocate_at_least()  (since C++23)
n - number of objects earlier passed to allocate() , or a number between requested and actually allocated number of objects via allocate_at_least() (may be equal to either bound) (since C++23)

Return value

(none)

Example

#include <algorithm>
#include <cstddef>
#include <iostream>
#include <memory>
#include <string>
 
class S {
    inline static int n{1};
    int m{};
    void pre() const { std::cout << "#" << m << std::string(m, ' '); }
public:
    S(int x) : m{n++} { pre(); std::cout << "S::S(" << x << ");\n"; }
    ~S() { pre(); std::cout << "S::~S();\n"; }
    void id() const { pre(); std::cout << "S::id();\n"; }
};
 
int main() {
    constexpr std::size_t n{4};
    std::allocator<S> allocator;
    try {
        S* s = allocator.allocate(n); // may throw
        for (std::size_t i{}; i != n; ++i) {
        //  allocator.construct(&s[i], i+42); // removed in C++20
            std::construct_at(&s[i], i+42); // since C++20
        }
        std::for_each_n(s, n, [](const auto& e) { e.id(); });
        std::destroy_n(s, n);
        allocator.deallocate(s, n);
    }
    catch(std::bad_array_new_length const& ex) { 
        std::cout << ex.what() << '\n'; }
    catch(std::bad_alloc const& ex) { 
        std::cout << ex.what() << '\n'; }
}

Result

#1 S::S(42);
#2  S::S(43);
#3   S::S(44);
#4    S::S(45);
#1 S::id();
#2  S::id();
#3   S::id();
#4    S::id();
#1 S::~S();
#2  S::~S();
#3   S::~S();
#4    S::~S();