这个问题是关于在插入std :: map期间构造自定义分配器的实例。
以下是std::map<int,int>的自定义分配器以及使用它的小程序：
#include <stddef.h>
#include <stdio.h>
#include <map>
#include <typeinfo>
class MyPool {
public:
void * GetNext() {
return malloc(24);
}
void Free(void *ptr) {
free(ptr);
}
};
template<typename T>
class MyPoolAlloc {
public:
static MyPool *pMyPool;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
typedef T* pointer;
typedef const T* const_pointer;
typedef T& reference;
typedef const T& const_reference;
typedef T value_type;
template<typename X>
struct rebind
{ typedef MyPoolAlloc<X> other; };
MyPoolAlloc() throw() {
printf("-------Alloc--CONSTRUCTOR--------%08x %32s\n", this, typeid(T).name());
}
MyPoolAlloc(const MyPoolAlloc&) throw() {
printf(" Copy Constructor ---------------%08x %32s\n", this, typeid(T).name());
}
template<typename X>
MyPoolAlloc(const MyPoolAlloc<X>&) throw() {
printf(" Construct T Alloc from X Alloc--%08x %32s %32s\n", this, typeid(T).name(), typeid(X).name());
}
~MyPoolAlloc() throw() {
printf(" Destructor ---------------------%08x %32s\n", this, typeid(T).name());
};
pointer address(reference __x) const { return &__x; }
const_pointer address(const_reference __x) const { return &__x; }
pointer allocate(size_type __n, const void * hint = 0) {
if (__n != 1)
perror("MyPoolAlloc::allocate: __n is not 1.\n");
if (NULL == pMyPool) {
pMyPool = new MyPool();
printf("======>Creating a new pool object.\n");
}
return reinterpret_cast<T*>(pMyPool->GetNext());
}
//__p is not permitted to be a null pointer
void deallocate(pointer __p, size_type __n) {
pMyPool->Free(reinterpret_cast<void *>(__p));
}
size_type max_size() const throw() {
return size_t(-1) / sizeof(T);
}
void construct(pointer __p, const T& __val) {
printf("+++++++ %08x %s.\n", __p, typeid(T).name());
::new(__p) T(__val);
}
void destroy(pointer __p) {
printf("-+-+-+- %08x.\n", __p);
__p->~T();
}
};
template<typename T>
inline bool operator==(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) {
return true;
}
template<typename T>
inline bool operator!=(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) {
return false;
}
template<typename T>
MyPool* MyPoolAlloc<T>::pMyPool = NULL;
int main(int argc, char *argv[]) {
std::map<int, int, std::less<int>, MyPoolAlloc<std::pair<const int,int> > > m;
//random insertions in the map
m.insert(std::pair<int,int>(1,2));
m[5] = 7;
m[8] = 11;
printf("======>End of map insertions.\n");
return 0;
}
以下是该计划的输出：
-------Alloc--CONSTRUCTOR--------bffcdaa6 St4pairIKiiE Construct T Alloc from X Alloc--bffcda77 St13_Rb_tree_nodeISt4pairIKiiEE St4pairIKiiE Copy Constructor ---------------bffcdad8 St13_Rb_tree_nodeISt4pairIKiiEE Destructor ---------------------bffcda77 St13_Rb_tree_nodeISt4pairIKiiEE Destructor ---------------------bffcdaa6 St4pairIKiiE ======>Creating a new pool object. Construct T Alloc from X Alloc--bffcd9df St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d028 St4pairIKiiE. Destructor ---------------------bffcd9df St4pairIKiiE Construct T Alloc from X Alloc--bffcd95f St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d048 St4pairIKiiE. Destructor ---------------------bffcd95f St4pairIKiiE Construct T Alloc from X Alloc--bffcd95f St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE +++++++ 0985d068 St4pairIKiiE. Destructor ---------------------bffcd95f St4pairIKiiE ======>End of map insertions. Construct T Alloc from X Alloc--bffcda23 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d068. Destructor ---------------------bffcda23 St4pairIKiiE Construct T Alloc from X Alloc--bffcda43 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d048. Destructor ---------------------bffcda43 St4pairIKiiE Construct T Alloc from X Alloc--bffcda43 St4pairIKiiE St13_Rb_tree_nodeISt4pairIKiiEE -+-+-+- 0985d028. Destructor ---------------------bffcda43 St4pairIKiiE Destructor ---------------------bffcdad8 St13_Rb_tree_nodeISt4pairIKiiEE
输出的最后两列显示每次插入地图时都会构造std::pair<const int, int>的分配器。为什么这有必要？有没有办法压制这个？
谢谢！
编辑：此代码在x86机器上使用g ++版本4.1.2进行了测试。如果您希望在64位计算机上运行它，则必须至少更改行return malloc(24)。更改为return malloc(48)应该有效。
答案 0 :(得分：1)
之所以如此，是因为分配器是std::pair<const int, int>，但实现实际上需要分配一个更复杂的数据结构，其中包含一个成员。虽然我希望实际的分配器需要构建和缓存，但每次重构它都不是非法的。这是一个在不改变实现的情况下无法逃脱的实现细节。创建的实际分配器类型是St13_Rb_tree_nodeISt4pairIKiiEE（错位名称）。
答案 1 :(得分：1)
在MyPool.h（单身）中：
class MyPool
{
...
public:
static MyPool & GetInstance( void );
private:
MyPool(void);
}
在MyPool.cpp中：
MyPool & MyPool::GetInstance( void )
{
static MyPool retval;
return retval;
}
在fooStdAllocator.h中：
#pragma once
#include "MyPool.h"
#pragma push_macro( "new" )
#undef new
#include <new>
template <class T1> class fooStdAllocator;
// Description:
// Specialize for void
template <> class fooStdAllocator<void>
{
public:
typedef void * pointer;
typedef const void* const_pointer;
typedef void value_type;
template <class U1> struct rebind { typedef fooStdAllocator<U1> other; };
};
template <class T1> class fooStdAllocator
{
public:
// Description:
// Typedefs
typedef T1 value_type;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
typedef T1* pointer;
typedef const T1* const_pointer;
typedef T1& reference;
typedef const T1& const_reference;
// Description:
// The rebind member allows a container to construct an allocator for some arbitrary type out of
// the allocator type provided as a template parameter.
template <class U1> struct rebind { typedef fooStdAllocator<U1> other; };
// Description:
// Constructors
fooStdAllocator( void ) : pool(MyPool::GetInstance()) {};
fooStdAllocator( const fooStdAllocator& other ) : pool(MyPool::GetInstance()) {};
template <class U1> fooStdAllocator(const fooStdAllocator<U1>&) : pool(MyPool::GetInstance()) {};
// Description:
// Destructor
~fooStdAllocator( void ) {};
// Description:
// Returns the address of r as a pointer type. This function and the following function are used
// to convert references to pointers.
pointer address(reference r) const { return &r; };
const_pointer address(const_reference r) const { return &r; };
// Description:
// Allocate storage for n values of T1.
pointer allocate( size_type n, fooStdAllocator<void>::const_pointer hint = 0 )
{
// I would never do it that way:
//pointer return_value = reinterpret_cast<pointer>( pool.GetNext() );
// I would prefer to use the got size to allocate:
pointer return_value = reinterpret_cast<pointer>( pool.GetNext(n) );
if ( return_value == 0 )
throw std::bad_alloc();
return return_value;
};
// Description:
// Deallocate storage obtained by a call to allocate.
void deallocate(pointer p, size_type n)
{
pool.Free(p);
};
// Description:
// Return the largest possible storage available through a call to allocate.
size_type max_size() const
{
size_type return_value = 0xFFFFFFFF;
return_value /= sizeof(T1);
return return_value;
};
// Description:
// Construct an object of type T1 at the location of ptr
void construct(pointer ptr)
{
::new (reinterpret_cast<void*>(ptr)) T1;
};
// Description:
// Construct an object of type T1 at the location of ptr, using the value of U1 in the call to the
// constructor for T1.
template <class U1> void construct(pointer ptr, const U1& val)
{
::new (reinterpret_cast<void*>(ptr)) T1(val);
};
// Description:
// Construct an object of type T1 at the location of ptr, using the value of T1 in the call to the
// constructor for T1.
void construct(pointer ptr, const T1& val)
{
::new (reinterpret_cast<void*>(ptr)) T1(val);
};
// Description:
// Call the destructor on the value pointed to by p
void destroy(pointer p)
{
p->T1::~T1();
};
private:
MyPool &pool;
};
// Return true if allocators b and a can be safely interchanged. "Safely interchanged" means that b could be
// used to deallocate storage obtained through a and vice versa.
template <class T1, class T2> bool operator == ( const fooStdAllocator<T1>& a, const fooStdAllocator<T2>& b)
{
return true;
};
// Return false if allocators b and a can be safely interchanged. "Safely interchanged" means that b could be
// used to deallocate storage obtained through a and vice versa.
template <class T1, class T2> bool operator != ( const fooStdAllocator<T1>& a, const fooStdAllocator<T2>& b)
{
return false;
};
#pragma pop_macro( "new" )
您可以按如下方式使用它：
std::map<keyT,valueT,std::less<keyT>,fooStdAllocator> your_map;