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要使用make_shared在每个线程中非常频繁地创建和销毁，而shared_ptr将不会传递给另一个线程，在这种情况下，我决定使用boost :: pool编写自定义allocate_shared分配器作为其成员根据类型分配固定大小的内存。

我的代码如下：

ObjectAllocator.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;
}

main.cpp中：

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;
}

当我尝试测试是否，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。

调试器在shared_ptr_base.h中说seg错误

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

当我尝试使boost :: pool静态时，它在单线程中工作正常并在多线程中崩溃调试器在shared_ptr_base.h中说seg错误

：_M_use_count（1），_ M_weak_count（1）{}

---

更新：我使boost :: pool成为静态thread_local，它现在正常工作

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的副本构造函数每次调用时都会创建一个新的boost::pool实例。

由于std::allocate_shared复制分配器（cppreference），用于分配ObjectAllocator的std::shared_ptr的实例在shared_ptr被销毁之前被它的池破坏。

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

可能与你的问题无关，但也有其他一些问题：

• 你不加入thread1的main。这会调用std::terminate并让你失败。
• boost::pool<> ObjectAllocator<T>::pool_(block_size); - ObjectAllocator<T>::部分是多余的和非标准的。 （仅在MSVC中接受afaik）