c++ - 尝试编写自定义 allocate_shared 分配器并使其成为 thread_local 时崩溃

Question

我的程序有几种类型的小对象在使用 make_shared 的每个线程中非常频繁地创建和销毁，并且 shared_ptr 不会传递给另一个线程，在这种情况下，我决定编写一个带有 boost 的自定义 allocate_shared 分配器::pool 作为其成员，根据类型分配固定大小的内存。

我的代码如下:

对象分配器.h:

#include <boost/pool/pool.hpp>

template<typename T>
class ObjectAllocator
{
public:
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;
    typedef T* pointer;
    typedef const T* const_pointer;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T value_type;

    auto static constexpr block_size=64+sizeof(value_type);

public:
    ObjectAllocator() noexcept:pool_(block_size){}
    ObjectAllocator(const ObjectAllocator &other) noexcept :pool_(block_size){}
    ~ObjectAllocator()=default;

    template<typename U>
    ObjectAllocator(const ObjectAllocator<U> &other) noexcept :pool_(block_size){}

    template<typename U>
    ObjectAllocator& operator= (const ObjectAllocator<U> &other){
        return *this;
    }

    ObjectAllocator<T>& operator = (const ObjectAllocator &other){
        return *this;
    }

    template<typename U>
    struct rebind{ typedef ObjectAllocator<U> other; };

    T *allocate(size_type n, const void *hint=nullptr){
#ifdef _DEBUG
        assert(n==1);
#endif
        return static_cast<T*>(pool_.malloc());
    }

    void deallocate(T *ptr, size_type n){
#ifdef _DEBUG
        assert(n==1);
#endif
        pool_.free(ptr);
    }

private:
    boost::pool<> ObjectAllocator<T>::pool_(block_size);
}

template<typename T, typename U>
inline bool operator == (const ObjectAllocator<T>&, const ObjectAllocator<U>&){
    return true;
}

template<typename T, typename U>
inline bool operator != (const ObjectAllocator<T>& a, const ObjectAllocator<U> &b){
    return !(a==b);
}


namespace Allocator {
template <typename T>
thread_local ObjectAllocator<T> allocator;
}

主要.cpp:

class ObjectA{
public:
    int s=0;
    void func(){
        std::cout<<s<<std::endl;
    }
    ObjectA() {//std::cout<<"()"<<std::endl;}
    ~ObjectA() {//std::cout<<"~"<<std::endl;}
};

std::vector<std::shared_ptr<ObjectA>> vec;
void test(){
    static uint32_t loop_count=1000*1000;
    for(uint32_t i=0;i<loop_count;i++){
         shared_ptr<ObjectA> packet = allocate_shared<ObjectA, ObjectAllocator<ObjectA>>(Allocator::allocator<ObjectA>);
         vec.push_back(packet);
    }
    vec.clear();
}

std::vector<std::shared_ptr<ObjectA>> vec2;
void test2(){
    static uint32_t loop_count=1000*1000;
    for(uint32_t i=0;i<loop_count;i++){
        shared_ptr<ObjectA> packet = allocate_shared<ObjectA, ObjectAllocator<ObjectA>>(Allocator::allocator<ObjectA>);
        vec2.push_back(packet);
    }
    vec2.clear();
}

int main() {
    std::thread thread1(test);
    test2();
    return 0;
}

当我尝试测试它时，它崩溃了，我不知道为什么。任何人都可以帮助使其正确吗？提前致谢。

调试器说 shared_ptr_base.h 中有段错误

void* _M_get_deleter(const std::type_info& __ti) const noexcept { return _M_pi ? _M_pi->_M_get_deleter(__ti) : nullptr; }

当我尝试将 boost::pool 设置为静态时，它在单线程中工作正常而在多线程中崩溃调试器说 shared_ptr_base.h 中有段错误

: _M_use_count(1), _M_weak_count(1) { }

---

更新:我将 boost::pool 设置为静态 thread_local，它现在可以正常工作了

template<typename T>
class ObjectAllocator
{
public:
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;
    typedef T* pointer;
    typedef const T* const_pointer;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T value_type;

    auto static constexpr block_size=64+sizeof(value_type);

public:
    ObjectAllocator() noexcept{}
    ObjectAllocator(const ObjectAllocator &other) noexcept {}
    ~ObjectAllocator()=default;

    template<typename U>
    ObjectAllocator(const ObjectAllocator<U> &other) noexcept {}

    template<typename U>
    ObjectAllocator& operator= (const ObjectAllocator<U> &other){
        return *this;
    }

    ObjectAllocator<T>& operator = (const ObjectAllocator &other){
        return *this;
    }

    template<typename U>
    struct rebind{ typedef ObjectAllocator<U> other; };

    T *allocate(size_type n, const void *hint=nullptr){
#ifdef _DEBUG
        assert(n==1);
#endif
        return static_cast<T*>(pool_.malloc());
    }

    void deallocate(T *ptr, size_type n){
#ifdef _DEBUG
        assert(n==1);
#endif
        pool_.free(ptr);
    }

private:
    thread_local static boost::pool<> pool_;
};

template<typename T>
thread_local boost::pool<> ObjectAllocator<T>::pool_(block_size);


template<typename T, typename U>
inline bool operator == (const ObjectAllocator<T>&, const ObjectAllocator<U>&){
    return true;
}

template<typename T, typename U>
inline bool operator != (const ObjectAllocator<T>& a, const ObjectAllocator<U> &b){
    return !(a==b);
}

namespace Allocator {
template <typename T>
thread_local static ObjectAllocator<T> allocator;
}

template <typename T, typename ...Args>
inline auto custom_make_shared(Args... args){
    return std::allocate_shared<T,ObjectAllocator<T>>(Allocator::allocator<T>,std::forward<Args>(args)...);
}

Answer 1

ObjectAllocator 的复制构造函数创建 boost::pool 的新实例每次他们被调用时。

作为std::allocate_shared复制分配器(cppreference)，ObjectAllocator 的实例用于分配 std::shared_ptr在 shared_ptr 之前被它的池破坏被摧毁。

相关问题:C++ stateful allocator de-allocate issues

可能与你的问题无关，但也有一些其他问题:

• 你不加入thread1在main .这将调用 std::terminate让你的程序崩溃。
• boost::pool<> ObjectAllocator<T>::pool_(block_size); - ObjectAllocator<T>::部分是多余的和不标准的。 (afaik 仅在 MSVC 中接受)