monotonic_allocator.hh

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#ifndef MEMORY_RESOURCE
#define MEMORY_RESOURCE
 
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <memory>
#include <new>
 
// A minimal re-implementation of std::monotonic_buffer_resource.
// Because libc++ doesn't have this yet, even though it's part of c++17.
 
class monotonic_allocator
{
public:
        monotonic_allocator() = default;
 
        monotonic_allocator(size_t initialSize)
                : nextSize(initialSize)
        {
                assert(initialSize != 0);
        }
 
        monotonic_allocator(void* buffer, size_t bufferSize)
                : current(buffer)
                , available(bufferSize)
                , nextSize(2 * bufferSize)
        {
                assert(buffer || bufferSize == 0);
        }
 
        monotonic_allocator(const monotonic_allocator&) = delete;
        monotonic_allocator& operator=(const monotonic_allocator&) = delete;
 
        ~monotonic_allocator()
        {
                void* p = head;
                while (p) {
                        void* next = *static_cast<void**>(p);
                        free(p);
                        p = next;
                }
        }
 
        [[nodiscard]] void* allocate(size_t bytes, size_t alignment)
        {
                assert(alignment <= alignof(max_align_t));
 
                if (bytes == 0) bytes = 1;
                void* p = std::align(alignment, bytes, current, available);
                if (!p) {
                        newBuffer(bytes);
                        p = current;
                }
                current = static_cast<char*>(current) + bytes;
                available -= bytes;
                return p;
        }
 
private:
        void newBuffer(size_t bytes)
        {
                size_t n = std::max(nextSize, bytes);
                void* newBuf = malloc(n + sizeof(void*));
                if (!newBuf) {
                        throw std::bad_alloc();
                }
 
                void** p = static_cast<void**>(newBuf);
                p[0] = head;
 
                current = static_cast<void*>(&p[1]);
                available = n;
                nextSize = 2 * n;
                head = newBuf;
        }
 
private:
        void* current = nullptr;
        size_t available = 0;
        size_t nextSize = 1024;
        void* head = nullptr;
};
 
#endif