stl auto_ptr源码注释及若干问题笔记
解决问题
1.注释 stl auto_ptr 源码。
2.弄明白 auto_ptr_ref 实现机制和要解决的问题；
3.自己实现一个auto_ptr，并实现一个测试程序;
stl auto_ptr 源码注释
{
/**
* A wrapper class to provide auto_ptr with reference semantics.
* For example, an auto_ptr can be assigned (or constructed from)
* the result of a function which returns an auto_ptr by value.
*
* All the auto_ptr_ref stuff should happen behind the scenes.
*/
//注释：当函数的返回值是 auto_ptr 时候，会造成一次传值转换。但是 auto_ptr构造
//时候，需要将传入的auto_ptr进行release(),因此无法以传值方式进行构造,必须是引用
//如： auto_ptr(auto_ptr& _a)。这就造成了函数返回值是传值时候会编译出错。
//出错场景：
//1. auto_ptr<int> p=new auto_ptr<int>(new int);
//2. auto_ptr<int> func1();
// auto_ptr<int> p=func1();
//
//解决方案是利用返回时候进行的一次强行值转换进行。
//1. return 时候，先强行将 auto_ptr 强行转换成 auto_ptr_ref;
//2. auto_ptr 增加 auto_ptr(auto_ptr_ref _ref) 构造实现。
//3. auto_ptr 增加一个操作符 operator = (auto_ptr_ref) 的实现。
//4. 至此,实现了对返回值是 auto_ptr 的值的函数调用；
template<typename _Tp1>
struct auto_ptr_ref
{
_Tp1* _M_ptr;
explicit
auto_ptr_ref(_Tp1* __p): _M_ptr(__p) { }
} _GLIBCXX_DEPRECATED;
/**
* @brief A simple smart pointer providing strict ownership semantics.
*
* The Standard says:
* <pre>
* An @c auto_ptr owns the object it holds a pointer to. Copying
* an @c auto_ptr copies the pointer and transfers ownership to the
* destination. If more than one @c auto_ptr owns the same object
* at the same time the behavior of the program is undefined.
*
* The uses of @c auto_ptr include providing temporary
* exception-safety for dynamically allocated memory, passing
* ownership of dynamically allocated memory to a function, and
* returning dynamically allocated memory from a function. @c
* auto_ptr does not meet the CopyConstructible and Assignable
* requirements for Standard Library <a
* href="tables.html#65">container</a> elements and thus
* instantiating a Standard Library container with an @c auto_ptr
* results in undefined behavior.
* </pre>
* Quoted from [20.4.5]/3.
*
* Good examples of what can and cannot be done with auto_ptr can
* be found in the libstdc++ testsuite.
*
* _GLIBCXX_RESOLVE_LIB_DEFECTS
* 127. auto_ptr<> conversion issues
* These resolutions have all been incorporated.
*/
template<typename _Tp>
class auto_ptr
{
private:
//保存指针的数据
_Tp* _M_ptr;
public:
/// The pointed-to type.
//也许是为了增加可读性
//手册中规定了 auto_ptr::element_type 是类型别名
typedef _Tp element_type;
/**
* @brief An %auto_ptr is usually constructed from a raw pointer.
* @param __p A pointer (defaults to NULL).
*
* This object now @e owns the object pointed to by @a __p.
*/
//explicit 这个限制符让强行转换失效，类如: auto_ptr<int> = 0x10001;
//指针 0x10001 不再构造 auto_ptr(*p) 并调用 operator =;
explicit
auto_ptr(element_type* __p = 0) throw() : _M_ptr(__p) { }
/**
* @brief An %auto_ptr can be constructed from another %auto_ptr.
* @param __a Another %auto_ptr of the same type.
*
* This object now @e owns the object previously owned by @a __a,
* which has given up ownership.
*/
//另一个相同类型的 auto_ptr 作为构造参数；
auto_ptr(auto_ptr& __a) throw() : _M_ptr(__a.release()) { }
/**
* @brief An %auto_ptr can be constructed from another %auto_ptr.
* @param __a Another %auto_ptr of a different but related type.
*
* A pointer-to-Tp1 must be convertible to a
* pointer-to-Tp/element_type.
*
* This object now @e owns the object previously owned by @a __a,
* which has given up ownership.
*/
//另一个不同类型的 auto_ptr 作为构造参数;
template<typename _Tp1>
auto_ptr(auto_ptr<_Tp1>& __a) throw() : _M_ptr(__a.release()) { }
/**
* @brief %auto_ptr assignment operator.
* @param __a Another %auto_ptr of the same type.
*
* This object now @e owns the object previously owned by @a __a,
* which has given up ownership. The object that this one @e
* used to own and track has been deleted.
*/
//相同类型的 auto_ptr 赋值
//赋值的时候，先将右值 auto_ptr 保存的地址释放掉，
//左值 auto_ptr 赋值，注意返回是自己的一个引用。
auto_ptr&
operator=(auto_ptr& __a) throw()
{
reset(__a.release());
return *this;
}
/**
* @brief %auto_ptr assignment operator.
* @param __a Another %auto_ptr of a different but related type.
*
* A pointer-to-Tp1 must be convertible to a pointer-to-Tp/element_type.
*
* This object now @e owns the object previously owned by @a __a,
* which has given up ownership. The object that this one @e
* used to own and track has been deleted.
*/
//不相同类型的 auto_ptr 赋值
//赋值的时候，先将右值 auto_ptr 保存的地址释放掉，
//左值 auto_ptr 赋值，注意返回是自己的一个引用。
template<typename _Tp1>
auto_ptr&
operator=(auto_ptr<_Tp1>& __a) throw()
{
reset(__a.release());
return *this;
}
/**
* When the %auto_ptr goes out of scope, the object it owns is
* deleted. If it no longer owns anything (i.e., @c get() is
* @c NULL), then this has no effect.
*
* The C++ standard says there is supposed to be an empty throw
* specification here, but omitting it is standard conforming. Its
* presence can be detected only if _Tp::~_Tp() throws, but this is
* prohibited. [17.4.3.6]/2
*/
//智能指针析构时候，释放自己拥有的申请的内存
~auto_ptr() { delete _M_ptr; }
/**
* @brief Smart pointer dereferencing.
*
* If this %auto_ptr no longer owns anything, then this
* operation will crash. (For a smart pointer, <em>no longer owns
* anything</em> is the same as being a null pointer, and you know
* what happens when you dereference one of those...)
*/
//智能指针提领返回类型的引用
//注意，如果这里如果智能指针是空值，程序会挂。
//这里的const是指不能修改指针，对于指针指向的对象，是可以修改的。
element_type&
operator*() const throw()
{
_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
return *_M_ptr;
}
/**
* @brief Smart pointer dereferencing.
*
* This returns the pointer itself, which the language then will
* automatically cause to be dereferenced.
*/
//操作符 -> 会返回指针
element_type*
operator->() const throw()
{
_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
return _M_ptr;
}
/**
* @brief Bypassing the smart pointer.
* @return The raw pointer being managed.
*
* You can get a copy of the pointer that this object owns, for
* situations such as passing to a function which only accepts
* a raw pointer.
*
* @note This %auto_ptr still owns the memory.
*/
//返回原始指针，有一些函数可能使用原始指针，此时需要用到 get()
element_type*
get() const throw() { return _M_ptr; }
/**
* @brief Bypassing the smart pointer.
* @return The raw pointer being managed.
*
* You can get a copy of the pointer that this object owns, for
* situations such as passing to a function which only accepts
* a raw pointer.
*
* @note This %auto_ptr no longer owns the memory. When this object
* goes out of scope, nothing will happen.
*/
//放弃掉对 _M_ptr 的访问权限
element_type*
release() throw()
{
element_type* __tmp = _M_ptr;
_M_ptr = 0;
return __tmp;
}
/**
* @brief Forcibly deletes the managed object.
* @param __p A pointer (defaults to NULL).
*
* This object now @e owns the object pointed to by @a __p. The
* previous object has been deleted.
*/
//使用指针 p 对 _M_ptr 进行赋值
void reset(element_type* __p = 0) throw()
{
if (__p != _M_ptr)
{
delete _M_ptr;
_M_ptr = __p;
}
}
/**
* @brief Automatic conversions
*
* These operations convert an %auto_ptr into and from an auto_ptr_ref
* automatically as needed. This allows constructs such as
* @code
* auto_ptr<Derived> func_returning_auto_ptr(.....);
* ...
* auto_ptr<Base> ptr = func_returning_auto_ptr(.....);
* @endcode
*/
//使用见 auto_ptr_ref 的设计方法
//使用 auto_ptr_ref 构造 auto_ptr
auto_ptr(auto_ptr_ref<element_type> __ref) throw()
: _M_ptr(__ref._M_ptr) { }
//使用见 auto_ptr_ref 的设计方法
//使用 auto_ptr_ref 复制
auto_ptr&
operator=(auto_ptr_ref<element_type> __ref) throw()
{
if (__ref._M_ptr != this->get())
{
delete _M_ptr;
_M_ptr = __ref._M_ptr;
}
return *this;
}
//使用见 auto_ptr_ref 的设计方法,
//类型强行转换,将自己（auto_ptr）强行转成 auto_ptr_ref;
//转换之后，自己不再拥有内存
template<typename _Tp1>
operator auto_ptr_ref<_Tp1>() throw()
{ return auto_ptr_ref<_Tp1>(this->release()); }
//类型强行转换,将自己（auto_ptr）强行转成另一个类型的auto_ptr;
//转换之后，自己不再拥有内存
//这个在什么场景下调用？ 有些费解。
//TODO:解析这个函数应用场景
template<typename _Tp1>
operator auto_ptr<_Tp1>() throw()
{ return auto_ptr<_Tp1>(this->release()); }
} _GLIBCXX_DEPRECATED;
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 541. shared_ptr template assignment and void
// 对空类型的 auto_ptr 偏特化,如果不做处理, auto_ptr<void> p4; 会出错
template<>
class auto_ptr<void>
{
public:
//手册中规定了 auto_ptr::element_type 是类型别名
typedef void element_type;
} _GLIBCXX_DEPRECATED;
//C++ 11 宏
#if __cplusplus >= 201103L
template<_Lock_policy _Lp>
template<typename _Tp>
inline
__shared_count<_Lp>::__shared_count(std::auto_ptr<_Tp>&& __r)
: _M_pi(new _Sp_counted_ptr<_Tp*, _Lp>(__r.get()))
{ __r.release(); }
template<typename _Tp, _Lock_policy _Lp>
template<typename _Tp1>
inline
__shared_ptr<_Tp, _Lp>::__shared_ptr(std::auto_ptr<_Tp1>&& __r)
: _M_ptr(__r.get()), _M_refcount()
{
__glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
static_assert( sizeof(_Tp1) > 0, "incomplete type" );
_Tp1* __tmp = __r.get();
_M_refcount = __shared_count<_Lp>(std::move(__r));
__enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
}
//C++ 新增 share_ptr
//gcc.4.9.3/gcc-4.9.3/libstdc++-v3/include/bits/shared_ptr_base.h
template<typename _Tp>
template<typename _Tp1>
inline
shared_ptr<_Tp>::shared_ptr(std::auto_ptr<_Tp1>&& __r)
: __shared_ptr<_Tp>(std::move(__r)) { }
//C++ 新增 unique_ptr,关于 unique_ptr 在文件
//gcc.4.9.3/gcc-4.9.3/libstdc++-v3/include/bits/unique_ptr.h
template<typename _Tp, typename _Dp>
template<typename _Up, typename>
inline
unique_ptr<_Tp, _Dp>::unique_ptr(auto_ptr<_Up>&& __u) noexcept
: _M_t(__u.release(), deleter_type()) { }
#endif
_GLIBCXX_END_NAMESPACE_VERSION
} // namespaceauto_ptr_ref 实现机制
auto_ptr_ref 是用来解决 auto_ptr::auto_ptr(const auto_ptr&);
类似这种场景：
auto_ptr function();
int main(){auto_ptr f=function();}注意到，这个 function 的返回值是一个临时变量(这个匿名的变量会在这个语句结束之后自动释放（自动析构）。C++语法规定，临时变量传输必须是 const 引用或者传值.这样做是为了避免程序员修改一个即将被销毁的变量。
那如何根据 function 的返回值来生成一个 'f' 呢？答案是通过利用 auto_ptr_ref 类。 C++编译器通过以下两个过程达成目标。
1.将 auto_ptr 转成一个 auto_ptr_ref 对象， 这时候 auto_ptr 将自身的指向的内存传给 auto_ptr_ref.(注意，这一步我们是通过调用一个 no-const 强行转换函数临时生成).
- 将 auto_ptr_ref 转成 auto_ptr, 这时候的 auto_ptr 就是我们所需要的'f1'，并获得 auto_ptr_ref 指向的内存。
这里面的返回值优化和函数中的const 参数调用重载另外说明
下面的 function1(),function2() 可以清楚的观察到这个过程。
#include <iostream>
using std::cout;
using std::endl;
class auto_ptr
{
private:
struct auto_ptr_ref { };
public:
auto_ptr() { cout << "auto_ptr::auto_ptr()\n"; }
auto_ptr(auto_ptr&) { cout << "auto_ptr::auto_ptr(auto_ptr&)\n"; }
operator auto_ptr_ref()
{ cout << "auto_ptr::operator auto_ptr_ref()\n"; }
auto_ptr(auto_ptr_ref) { cout <<
"auto_ptr::auto_ptr(auto_ptr_ref)\n"; }
~auto_ptr() { cout << "auto_ptr::~auto_ptr()\n"; }
};
auto_ptr function1();
auto_ptr function2();
int main()
{
auto_ptr f1=function1(); // 调用 function function1() ,过程如下
// 调用 "auto_ptr::auto_ptr(const auto_ptr&)", 没找到
// 不能调用"auto_ptr::auto_ptr(auto_ptr&)",因为function1()的返回值是一个临时变量。
// 编译器决定执行“两步转换”:
// 1) 使用 "auto_ptr::operator auto_ptr_ref()"
// 2) 使用 "auto_ptr::auto_ptr(auto_ptr_ref)" 构造出 'f1'
// 此时，使用 "auto_ptr::~auto_ptr()" 析构销毁 'function1()' 的返回值
cout << "after1\n";
auto_ptr f2=function2(); //返回值步骤具体和function1()类似,但是里面还生成了一个匿名函数，因此会多一次 “两步转换”;
cout << "after2\n";
} // 2 calls to "auto_ptr::~auto_ptr()" to destroy 'f1' and 'f2'
auto_ptr function1() //编译器**如果没执行返回值优化**,其调用如下所示
{
auto_ptr a; // 调用"auto_ptr::auto_ptr()",生成变量 a
return a; // 调用"auto_ptr::auto_ptr(auto_ptr&)" 生成临时变量返回。
} // 调用"auto_ptr::~auto_ptr()" 销毁变量 'a'
auto_ptr function2()
{
return auto_ptr(); //调用"auto_ptr::auto_ptr()"生成临时变量。
//调用 "auto_ptr::auto_ptr(const auto_ptr&)",没找到
//编译器质性“两步转换”,生成一个临时变量用于返回。
}
//调用"auto_ptr::~auto_ptr()" 销毁返回的临时变量。
lawrencechi@LawrencechideMacBook-Pro ~/temp/test » make 127 ↵
g++ -g main.cpp -o test
main.cpp:14:58: warning: control reaches end of non-void function [-Wreturn-type]
{ cout << "auto_ptr::operator auto_ptr_ref()\n"; }
^
1 warning generated.
lawrencechi@LawrencechideMacBook-Pro ~/temp/test » ./test
auto_ptr::auto_ptr()
auto_ptr::operator auto_ptr_ref()
auto_ptr::auto_ptr(auto_ptr_ref)
auto_ptr::~auto_ptr()
after1
auto_ptr::auto_ptr()
auto_ptr::operator auto_ptr_ref()
auto_ptr::auto_ptr(auto_ptr_ref)
auto_ptr::~auto_ptr()
auto_ptr::operator auto_ptr_ref()
auto_ptr::auto_ptr(auto_ptr_ref)
auto_ptr::~auto_ptr()
after2
auto_ptr::~auto_ptr()
auto_ptr::~auto_ptr()
lawrencechi@LawrencechideMacBook-Pro ~/temp/test »自己实现 auto_ptr
1.考虑构造函数 auto_ptr(const auto_ptr&) 不能用，必须用 auto_ptr(auto_ptr&)；
2.由于1，所以必须使用 auto_ptr_ref机制规避返回值为auto_ptr时候的问题;
3.必须要考虑特化版本 template<> auto_ptr
4.赋值操作符，构造函数考虑类型转换问题,构造函数中的 explicit 限定符问题；
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;
template<typename T>
struct auto_ptr_ref
{
T* pointee;
explicit
auto_ptr_ref(T* p): pointee(p) {}
};
template <class T>
class my_auto_ptr
{
public:
explicit my_auto_ptr(T* p=NULL):pointee(p) {
cout<<"my_auto_ptr(T* p=NULL)"<<pointee<<endl;
}
template <class U>
my_auto_ptr(my_auto_ptr<U>& rhs):pointee(rhs.release()){
cout<<"my_auto_ptr(my_auto_ptr<U>& rhs)"<<pointee<<endl;
}
~my_auto_ptr()
{
cout<<"~my_auto_ptr():"<<pointee<<endl;
if(pointee)
{
delete pointee;
pointee=NULL;
}
}
T* release()
{
T* tmp=this->pointee;
this->pointee=NULL;
return tmp;
}
template<class U>
void reset(U* rp)
{
if(pointee!=rp)
{
delete this->pointee;
pointee=rp;
}
}
T& operator*()const{ return *pointee;}
T* operator->()const{ return pointee;}
T* get() const {return pointee;}
template <class U>
my_auto_ptr<T>& operator=(my_auto_ptr<U>& rhs)
{
reset(rhs.release());
return *this;
}
my_auto_ptr& operator=(my_auto_ptr& rhs)
{
reset(rhs.release());
return *this;
}
my_auto_ptr(auto_ptr_ref<T> __ref): pointee(__ref.pointee) {
cout<<"my_auto_ptr(auto_ptr_ref<T> __ref)"<<endl;
}
my_auto_ptr&
operator=(auto_ptr_ref<T> __ref)
{
cout<<"my_auto_ptr& operator=(auto_ptr_ref<T> __ref)"<<endl;
if (__ref.pointee!= pointee)
{
delete pointee;
pointee= __ref.pointee;
}
return *this;
}
template<typename _Tp1>
operator auto_ptr_ref<_Tp1>() throw()
{
cout<<"template<typename _Tp1> operator auto_ptr_ref<_Tp1>() throw()"<<endl;
return auto_ptr_ref<_Tp1>(this->release());
}
template<typename _Tp1>
operator my_auto_ptr<_Tp1>() throw()
{
cout<<"template<typename _Tp1>operator my_auto_ptr<_Tp1>() throw()"<<endl;
return my_auto_ptr<_Tp1>(this->release());
}
private:
T* pointee;
};
template<>
class my_auto_ptr<void>
{
public:
typedef void element_type;
};
//测试自动销毁
void func()
{
cout<<"func"<<endl;
my_auto_ptr<string> ps(new string("test"));
cout << *ps <<endl;
cout<< ps->size()<<endl;
}
//测试返回为 my_auto_ptr<int>
my_auto_ptr<int> func1()
{
my_auto_ptr<int> a(new int);
*a=12;
return a;
}
int main()
{
my_auto_ptr<int> p;
my_auto_ptr<int> p2;
my_auto_ptr<int> p3;
my_auto_ptr<void> p4; //test void
my_auto_ptr<void> p5;
std::cout << "before new" <<std::endl;
p = my_auto_ptr<int> (new int);
std::cout << "after new" <<std::endl;
std::cout << "before func" <<std::endl;
func();
std::cout << "after func" <<std::endl;
std::cout << "before func2" <<std::endl;
p2=func1();
std::cout << "after func2" <<std::endl;
*p = 11;
p2 = p;
std::cout << "p2 points to " << *p2 << '\n';
p4=p5;
return 0;
}
lawrencechi@LawrencechideMacBook-Pro ~/temp/test » g++ my_auto_ptr.cpp
lawrencechi@LawrencechideMacBook-Pro ~/temp/test » ./a.out
my_auto_ptr(T* p=NULL)0x0
my_auto_ptr(T* p=NULL)0x0
my_auto_ptr(T* p=NULL)0x0
before new
my_auto_ptr(T* p=NULL)0x7fdd6bd00000
template<typename _Tp1> operator auto_ptr_ref<_Tp1>() throw()
my_auto_ptr& operator=(auto_ptr_ref<T> __ref)
~my_auto_ptr():0x0
after new
before func
func
my_auto_ptr(T* p=NULL)0x7fdd6bd00010
test
4
~my_auto_ptr():0x7fdd6bd00010
after func
before func2
my_auto_ptr(T* p=NULL)0x7fdd6bd00030
template<typename _Tp1> operator auto_ptr_ref<_Tp1>() throw()
my_auto_ptr& operator=(auto_ptr_ref<T> __ref)
~my_auto_ptr():0x0
after func2
p2 points to 11
~my_auto_ptr():0x0
~my_auto_ptr():0x7fdd6bd00000
~my_auto_ptr():0x0