NAME¶

std::allocator::deallocate - std::allocator::deallocate

Synopsis¶

void deallocate( T* p, std::size_t n ); (constexpr since C++20)

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 (since C++20)
storage allocated within the evaluation of the same expression.

Parameters¶

pointer obtained from allocate()
p - or allocate_at_least()
(since C++23)
number of objects earlier passed to allocate()
n - , 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¶

// Run this code

#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'; }
}

Output:¶

#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();

See also¶

allocate allocates uninitialized storage
(public member function)
allocate_at_least allocates uninitialized storage at least as large as requested
(C++23) size
(public member function)
deallocate deallocates storage using the allocator
[static] (public static member function of std::allocator_traits<Alloc>)