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.