c – 如何将自定义分配器的完全相同状态传递给多个容器？

2023年5月8日 228次阅读

我正在编写一个分配器,它引用了某个类的另一个实例来跟踪分配的字节数.

下面是我正在尝试做的最小例子(改编自here),只是没有整个内存跟踪类,而是我引用了一些int来收集到目前为止分配的字节.此引用在main中分配,应传递给CustomAllocator：

#include <limits>   // numeric_limits
#include <iostream>
#include <typeinfo> // typeid

// container
#include <vector>
#include <list>
#include <forward_list>

template<typename T>
class CustomAllocator {
public:
    // type definitions
    typedef T value_type; /** Element type */
    typedef T* pointer; /** Pointer to element */
    typedef T& reference; /** Reference to element */
    typedef const T* const_pointer; /** Pointer to constant element */
    typedef const T& const_reference; /** Reference to constant element */
    typedef std::size_t size_type; /** Quantities of elements */
    typedef std::ptrdiff_t difference_type; /** Difference between two pointers */

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

    // return maximum number of elements that can be allocated
    size_type max_size () const throw() {
        return std::numeric_limits<std::size_t>::max() / sizeof(T);
    }

    CustomAllocator(std::size_t& memAllocated) :
            m_totalMemAllocated(memAllocated) {
        std::cout << "construct " << typeid(T).name() << std::endl;
    }

    CustomAllocator(const CustomAllocator& src) :
            m_totalMemAllocated(src.m_totalMemAllocated) {
        std::cout << "copy construct " << typeid(T).name() << std::endl;
    }

    template<class U>
    CustomAllocator(const CustomAllocator<U>& src) :
            m_totalMemAllocated(src.getTotalMemAllocated()) {
    }

    // allocate but don't initialize num elements of type T
    pointer allocate(size_type num, const void* = 0) {
        m_totalMemAllocated += num * sizeof(T);
        // print message and allocate memory with global new
        std::cout << "allocate " << num << " element(s)" << " of size "
                << sizeof(T) << std::endl;
        pointer ret = (pointer) (::operator new(num * sizeof(T)));
        std::cout << " allocated at: " << (void*) ret << std::endl;
        return ret;
    }

    // deallocate storage p of deleted elements
    void deallocate(pointer p, size_type num) {
        m_totalMemAllocated -= num * sizeof(T);
        // print message and deallocate memory with global delete
        std::cout << "deallocate " << num << " element(s)" << " of size "
                << sizeof(T) << " at: " << (void*) p << std::endl;
        ::operator delete((void*) p);
    }




    // initialize elements of allocated storage p with value value
    // no need to call rebind with this variadic template anymore in C++11
    template<typename _U, typename ... _Args>
    void construct(_U* p, _Args&&... args) {
        ::new ((void *) p) _U(std::forward<_Args>(args)...);
    }

    // destroy elements of initialized storage p
    template<typename _U>
    void destroy(_U* p) {
        p->~_U();
    }

    // return address of values
    pointer address (reference value) const {
        return &value;
    }
    const_pointer address (const_reference value) const {
        return &value;
    }

private:
    std::size_t& m_totalMemAllocated;
};

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

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

int main() {

    std::size_t memAllocated = 0;

    CustomAllocator<int> allocatorInstance(memAllocated);

    std::vector<int> foo(allocatorInstance);
    foo.push_back(23);
    foo.push_back(12);
    foo.push_back(8);

    std::cout << "---" << std::endl;

    // here the same
    std::list<double> bar(allocatorInstance);
    bar.push_back(3.44);
    bar.push_back(1.18);
    bar.push_back(2.25);

    std::cout << "---" << std::endl;

    // debug output
    for (auto x : foo)
        std::cout << x << " ";
    for (auto x : bar)
        std::cout << x << " ";

    std::cout << "\nalloc_count: " << memAllocated << std::endl;

    std::cout << '\n';
    return 0;
}

我的问题在于我不知道如何将分配器实例的完全相同的状态(在示例m_totalMemAllocated中)传递给其他两个容器(这里：foo和bar).由于标准规定C 11分配器可以具有状态.

更新：

谢谢你到目前为止的答案:)

我知道您通常将CustomAllocators作为模板参数传递给std容器;像这样：

std::vector<int, CustomAllocator<int> > foo;
std::list<double, CustomAllocator<double> > bar;

另见：
Difference between allocator supplied as template parameter and allocator supplied as constructor argument in C++ containers?

但在这里我确实有一个我无法传递的状态,默认构造函数将被调用,除非我给引用一些默认值(但这不是我想要的),我不能使用它.

投入

std::size_t memAllocated = 0;

从main到全局范围将意味着使用CustomAllocator的所有容器最终将使用全局定义的memAllocated.但我想扩展它,以便我可以有一些额外的内存或实例memAllocated2然后再次分配给其他一些分配器实例.

边注：

有关与STD-Containers不同的容器的有状态版本的分配器,请参阅

How to track memory usage using EASTL?

最佳答案 为了确保在所有分配器实例之间共享状态,第一个想法是使其成为静态成员.但仅凭这一点是不够的,因为不同的模板实例确实是不同的类型,并且每个都有自己的静态成员副本.所以我只能想象两种方法：使状态成为仅包含静态成员的辅助类,或者使用单例模式：

辅助类的静态成员：

struct CustomAllocatorState {
    static std::size_t& m_totalMemAllocated;
}
std::size_t& CustomAllocatorState::m_totalMemAllocated = 0; # do not forget definition...

template<typename T>
class CustomAllocator {
public:
    ...
    pointer allocate(size_type num, const void* = 0) {
        CustomAllocatorState::m_totalMemAllocated += num * sizeof(T);
        ...

单例模式(你可以使用任何其他C单例模式,这一个很简单,但不能抵抗静态初始化惨败)：

class CustomAllocatorState {
    CustomAllocatorState(): m_val(0) {}
    static CustomAllocatorState state;

public:
    int m_val;
    static CustomAllocatorState& getState() {
        return state;
    }
};
CustomAllocatorState CustomAllocatorState::state;

template<typename T>
class CustomAllocator {
public:
    ...
    CustomAllocator() :
        state(CustomAllocatorState::getState()) {
            std::cout << "construct " << typeid(T).name() << std::endl;
    }
    ...
    pointer allocate(size_type num, const void* = 0) {
        state.m_totalMemAllocated += num * sizeof(T);
        ...
private:
    CustomAllocatorState& state;
};

辅助类中的静态成员可能更简单,但如果您已在应用程序中使用单例模式,那么在此处使用它也是有意义的.

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