关于自定义Allocator的问题？

Question

我想看一下STL容器类是如何管理其内存的，如何时分配、释放内存。于是我写了一个MyAllocator类，继承自std::allocator，所有的函数实现都是打印一行信息，然后调用相应的基类函数。代码如下：

#include <cstdio>
#include <cstring>
#include <string>
#include <vector>
#include <memory>


template <typename T>
class MyAllocator : public std::allocator<T> {
	typedef typename std::allocator<T> 			BaseType;
	typedef typename BaseType::size_type		size_type;
	typedef typename BaseType::pointer			pointer;
public:
	MyAllocator()
	{ printf( "MyAllocator Constructor.\n" );}
	MyAllocator( const MyAllocator &rhs )
	{printf( "MyAllocator Copy Constructor.\n" );}
	~MyAllocator()
	{ printf( "MyAllocator Destructor.\n" );}

	pointer allocate( size_type _Count, const void* _Hint )
	{
		printf( "MyAllocator::allocate() count = %u, hint = %x\n", _Count, _Hint );
		return BaseType::allocate( _Count, _Hint );
	}
	
	void deallocate( pointer _Ptr, size_type _Count	)
	{
		printf( "MyAllocator::deallocate() ptr = %x, count = %u\n", _Ptr, _Count );
		BaseType::deallocate( _Ptr, _Count );
	}	
	
	void construct( pointer _Ptr, const T &_Val )
	{
		printf( "MyAllocator::construct() ptr = %x\n", _Ptr );
		BaseType::construct( _Ptr, _Val );
	}

	void destroy( pointer _Ptr )
	{
		printf( "MyAllocator::destroy() ptr = %x\n", _Ptr );
		BaseType::destroy( _Ptr );
	}
};


struct Test {
	Test()
	{ printf( "Test Constructor.\n" ); }
	
	Test( const Test &rhs )
	{ printf( "Test Copy Constructor.\n" ); }	
	
	~Test()
	{ printf( "Test Destructor.\n" ); }	
	
	int x;
};

class MyString : public std::basic_string< char, std::char_traits<char>, MyAllocator<char> > {

};

template <typename T>
class MyVector : public std::vector< T, MyAllocator<T> > {

};

int main()
{	
	// MyVector test
	{
		MyVector<int> vec;
		vec.reserve(100);
		vec.push_back(1);
		vec.push_back(2);
		printf( "%u, %u\n", vec.size(), vec.capacity() );
	}

	// MyString test
	{
		MyString s;
		s.append("Hello, world!" );
	}
	
    return 0;
} 

本想在调用MyVector的reserve或push_back函数时， MyAllocator的allocate函数会被调用，会看到其输出信息，但是事与愿违。程序运行结果如下：

MyAllocator Constructor.
MyAllocator Copy Constructor.
MyAllocator Destructor.
MyAllocator Destructor.
2, 100

只有MyAllocator的构造和析构函数被调用，且都是因为MyVector的创建而引起，MyString虽然也指定了MyAllocator为其内存分配器，但并未引发MyAllocator的任何动作。

还有一点很奇怪，所有的MyAllocator对象都在调用reserve和push_back之前销毁了，似乎MyVector并未使用MyAllocator作为内存分配器。

这到底是什么原因呢？？？