Alloc, C++ (clang)

Language: Layout:

#include <iostream>

#ifndef SHORT_ALLOC_H
#define SHORT_ALLOC_H

// The MIT License (MIT)
// 
// Copyright (c) 2015 Howard Hinnant
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include <cstddef>
#include <cassert>

template <std::size_t N, std::size_t alignment = alignof(std::max_align_t)>
class arena {
    alignas(alignment) char buf_[N];
    char* ptr_;

public:
    ~arena() {ptr_ = nullptr;}
    arena() noexcept : ptr_(buf_) {}
    arena(const arena&) = delete;
    arena& operator=(const arena&) = delete;

template <std::size_t ReqAlign> char* allocate(std::size_t n);
    void deallocate(char* p, std::size_t n) noexcept;

static constexpr std::size_t size() noexcept {return N;}
    std::size_t used() const noexcept {return static_cast<std::size_t>(ptr_ - buf_);}
    void reset() noexcept {ptr_ = buf_;}

private:
    static std::size_t align_up(std::size_t n) noexcept {return (n + (alignment-1)) & ~(alignment-1);}
    bool pointer_in_buffer(char* p) noexcept {return buf_ <= p && p <= buf_ + N;}
};

template <std::size_t N, std::size_t alignment>
template <std::size_t ReqAlign>
char* arena<N, alignment>::allocate(std::size_t n) {
    static_assert(ReqAlign <= alignment, "alignment is too small for this arena");
    assert(pointer_in_buffer(ptr_) && "short_alloc has outlived arena");
    auto const aligned_n = align_up(n);
    if (static_cast<decltype(aligned_n)>(buf_ + N - ptr_) >= aligned_n) {
        char* r = ptr_;
        ptr_ += aligned_n;
        return r;
    }

static_assert(alignment <= alignof(std::max_align_t), "you've chosen an "
                  "alignment that is larger than alignof(std::max_align_t), and "
                  "cannot be guaranteed by normal operator new");
    return static_cast<char*>(::operator new(n));
}

template <std::size_t N, std::size_t alignment>
void arena<N, alignment>::deallocate(char* p, std::size_t n) noexcept {
    assert(pointer_in_buffer(ptr_) && "short_alloc has outlived arena");
    if (pointer_in_buffer(p)) {
        n = align_up(n);
        if (p + n == ptr_)
            ptr_ = p;
    } else {
        ::operator delete(p);
    }
}

template <class T, std::size_t N, std::size_t Align = alignof(std::max_align_t)>
class short_alloc {
public:
    using value_type = T;
    static auto constexpr alignment = Align;
    static auto constexpr size = N;
    using arena_type = arena<size, alignment>;

private:
    arena_type& a_;

public:
    short_alloc(const short_alloc&) = default;
    short_alloc& operator=(const short_alloc&) = delete;

short_alloc(arena_type& a) noexcept : a_(a) {
        static_assert(size % alignment == 0, "size N needs to be a multiple of alignment Align");
    }
    template <class U>
    short_alloc(const short_alloc<U, N, alignment>& a) noexcept : a_(a.a_) {}

template <class _Up> struct rebind {using other = short_alloc<_Up, N, alignment>;};

T* allocate(std::size_t n) {
        return reinterpret_cast<T*>(a_.template allocate<alignof(T)>(n*sizeof(T)));
    }
    void deallocate(T* p, std::size_t n) noexcept {
        a_.deallocate(reinterpret_cast<char*>(p), n*sizeof(T));
    }

template <class T1, std::size_t N1, std::size_t A1, class U, std::size_t M, std::size_t A2>
    friend bool operator==(const short_alloc<T1, N1, A1>& x, const short_alloc<U, M, A2>& y) noexcept;

template <class U, std::size_t M, std::size_t A> friend class short_alloc;
};

template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
inline bool operator==(const short_alloc<T, N, A1>& x, const short_alloc<U, M, A2>& y) noexcept {
    return N == M && A1 == A2 && &x.a_ == &y.a_;
}

template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
inline bool operator!=(const short_alloc<T, N, A1>& x, const short_alloc<U, M, A2>& y) noexcept {
    return !(x == y);
}

#endif  // SHORT_ALLOC_H

#include <iostream>
#include <new>
#include <vector>

// Replace new and delete just for the purpose of demonstrating that
//  they are not called.

std::size_t memory = 0;
std::size_t alloc = 0;

void* operator new(std::size_t s) throw(std::bad_alloc)
{
    memory += s;
    ++alloc;
    return malloc(s);
}

void  operator delete(void* p) throw()
{
    --alloc;
    free(p);
}

void memuse()
{
    std::cout << "memory = " << memory << '\n';
    std::cout << "alloc = " << alloc << '\n';
}

// Create a vector<T> template with a small buffer of 200 bytes.
//   Note for vector it is possible to reduce the alignment requirements
//   down to alignof(T) because vector doesn't allocate anything but T's.
//   And if we're wrong about that guess, it is a compile-time error, not
//   a run time error.
template <class T, std::size_t BufSize = 200>
using SmallVector = std::vector<T, short_alloc<T, BufSize, alignof(T)>>;

int main()
{
    // Create the stack-based arena from which to allocate
    SmallVector<int>::allocator_type::arena_type a;
    // Create the vector which uses that arena.
    SmallVector<int> v{a};
    // Exercise the vector and note that new/delete are not getting called.
    v.push_back(1);
    memuse();
    v.push_back(2);
    memuse();
    v.push_back(3);
    memuse();
    v.push_back(4);
    memuse();
    // Yes, the correct values are actually in the vector
    for (auto i : v)
        std::cout << i << ' ';
    std::cout << '\n';
    std::cout << "a " << sizeof(a) << '\n';
    std::cout << "v " << sizeof(v) << '\n';
}

Show compiler warnings [ + ] Compiler args [ + ] Show input