Allocator messing around

drop_and_reuse.cpp

#include <cstdio>
#include <memory_resource>
#include <new>
#include <type_traits>
#include <vector>
struct data {
	data() = default;
	data(int a) : a(a) {}
	~data() { printf("destructed\n"); }
	int a;
};
int main() {
	// Storage to hold the vector itself.
	std::aligned_storage_t<sizeof(std::pmr::vector<data>), alignof(std::pmr::vector<data>)> vector_holder;
	// Storage for the first 128 vector elements.
	std::aligned_storage_t<sizeof(data), alignof(data)> buffer[128];
	// Monotonic allocator.
        std::pmr::monotonic_buffer_resource alloc(buffer, 128*sizeof(data));
	// Placement new the vector in its local storage.
	std::pmr::vector<data>* v = new (&vector_holder) std::pmr::vector<data>(&alloc);
	v->reserve(128);
	v->emplace_back(1);
	v->emplace_back(1);
	v->emplace_back(1);
	v->emplace_back(1);
	alloc.release();
	v = new (&vector_holder) std::pmr::vector<data>(&alloc);
	v->reserve(128);
	v->emplace_back(2);
	v->emplace_back(2);
	v->emplace_back(2);
	v->emplace_back(2);
}

monotonic_allocator.cpp

#include <cstdio>
#include <memory_resource>
#include <new>
#include <type_traits>
#include <vector>
struct data {
        data() = default;
        data(int a) : a(a) {}
        // Never gets printed.
        ~data() { printf("destructed\n"); }
        int a;
};
int main() {
        // Storage to hold the vector itself.
        std::aligned_storage_t<sizeof(std::pmr::vector<data>),
                               alignof(std::pmr::vector<data>)>
            vector_holder;
        // Storage for the first 128 vector elements.
        std::aligned_storage_t<sizeof(data), alignof(data)> buffer[128];
        // Monotonic allocator.
        std::pmr::monotonic_buffer_resource alloc(buffer, 128 * sizeof(data));
        // Placement new the vector in its local storage.
        std::pmr::vector<data> *v =
            new (&vector_holder) std::pmr::vector<data>(&alloc);
        // Reserve all 128 elements. Their space is already allocated in buffer,
        // so no extra memory is required and prevents the vector from
        // reallocating when populating it. This allows us to take maximum
        // advantage of the buffer and avoids any moves...
        // untill thet 129th element is pushed.
        v->reserve(128);
        // Use emplace_back() to avoid temporaries.
        v->emplace_back(1);
        v->emplace_back(1);
        v->emplace_back(1);
        v->emplace_back(1);
}