假设我们有一些Foo对象允许：
cout << myFoo[3];
myFoo[5] = "bar";
这要求代理设计模式（详见Scott Meyers - link）
但现在让我们假设每个myFoo [i]也是一个Foo实例。
myFoo[7] = Foo{...};
myFoo[5] = "bar"; // Foo has a Foo(std::string) non-explicit constructor
我已接近实施，但我无法摆脱最后一个令人讨厌的“前向声明/不完整类型”错误。
首先，让我们轻松一点：
// x = someConstObject[4], so this must be Rvalue access
// i.e. someConstObject[4] = ... would be a contradiction / const violation
const Object operator[] (const Object& key) const {
return Object{ PyObject_GetItem(p, key.p) };
}
这是基本的非递归代理模式：
Proxy operator [] ( const Object& key ) { return Proxy{ *this, key }; }
class Proxy {
private:
const Object& container;
const Object& key;
public:
// at this moment we don't know whether it is 'container[key] = x' or 'x = container[key]'
Proxy( const Object& c, const Object& k ) : container{c}, key{k}
{ }
// Rvalue
// e.g. cout << myList[5]
operator Object() const {
return container[key]; // <-- invokes the original const [] overload
}
// Lvalue
// e.g. myList[5] = foo
const Object& operator= (const Object& rhs_ob) {
PyObject_SetItem( container.p, key.p, rhs_ob.p );
return rhs_ob; // allow daisy-chaining a = b = c etc.
}
#if 0
// I think this should come for free, as the above Rvalue handler
// ... collapses a Proxy into an Object
// e.g. myList[5] = someOtherList[7]
const Proxy& operator= (const Proxy& rhs) {
// Force resolution of rhs into Object
PyObject_SetItem( pContainerObj->p, pKeyObject->p, static_cast<Object>(rhs).p /* rhs.value->p*/ );
return rhs;
}
#endif
// ^ Note: allows:
// e.g. x = y[1] = z[2]; // <-- y[1] must return an Object
// e.g. if( y[1] = z[2] ) // <-- assigns and then checks that y[1] evaluates to true
};
不确定我是否需要最后一个处理程序：
无论如何，让它递归，我们需要
class Proxy : Object {
:
这意味着我们不能再在Object中定义Proxy，否则我们将得到“尝试从不完整类型建立”编译器错误。
让我们这样做。我们还必须修改构造函数以尽可能填写基类：
class Object::Proxy : public Object {
private:
const Object& container;
const Object& key;
public:
// at this moment we don't know whether it is 'c[k] = x' or 'x = c[k]'
// If it's 'c[k] = x', setting the base class to c[k] is going to
// either set it to the old value of c[k]
// or a None object (if it didn't have any value previously)
// we had better be certain to make sure the original c[k] overload
// returns None if unsuccessful
Proxy( const Object& c, const Object& k )
: container{c}, key{k}, Object{c[k]} // <-- might fail!
{ }
然后，由于Object基类，我们不再需要手动处理类型转换为对象：
// Rvalue
// e.g. cout << myList[5] hits 'const Object operator[]'
#if 0
// it looks as though we don't need to do this given that
// we now have Object as base class
operator Object() const {
return container[key];
}
#endif
但这就是它变得粗糙的地方。
如果我们将Object :: Proxy的定义移到（实际上是）之后的对象，原来的
Proxy operator [] ( const Object& key ) { return Proxy{ *this, key }; }
...现在给我们一个错误，因为我们使用了一个不完整的类（代理）。请注意，仅仅将定义移到外部并不能解决返回类型为Proxy的问题。如果它只是代理*我们可以做到。但代理不能。
它似乎是一个Catch-22，我看不到任何干净的解决方案。
有吗？
编辑：在回应暗示设计有缺陷的评论时，请记住Object是指针周围的轻量级包装器。它只有一个PyObject *数据成员。
编辑：我正在寻找的原始代码here
答案 0 :(得分：1)
你的前提似乎有缺陷。根据定义，Proxy 不是 Object;如果是，那么你首先不会称之为Proxy。然后你可以在没有代理的情况下解决你的问题，就像std::map之类的标准数据类型解决它一样：只需要operator[]在必要时返回对新创建的Object的引用。
您正在寻找类似std::vector<bool>代理模式的内容：operator[]返回Proxy operator=并隐式转换为非代理Object 1}}（对于你真正想要使用该值而不是分配给它的情况）。
class Object {
struct Proxy {
PyObject *container;
PyObject *key;
Proxy(PyObject *c, PyObject *k): container(c), key(k) {}
Proxy& operator= (const Object& value) {
PyObject_SetItem(container, key, value.p);
return *this;
}
operator Object() const {
PyObject *p = PyObject_GetItem(container, key);
if (p == nullptr) throw "proxy was not backed by a real object";
return p;
}
};
PyObject *p;
Object(PyObject* p): p(p) {}
public:
Object operator[] (const Object& key) const {
return PyObject_GetItem(p, key.p);
}
Proxy operator[] (const Object& key) { return {p, key.p}; }
};
答案 1 :(得分：0)
我最终解决了这个问题。
诀窍是简单地将该类用作自己的代理。
因此，最初Proxy对象提供转换以区分Lvalue和Rvalue访问，我只是将这些转换移回到我原来的Object类中：
mutable bool m_resolve_me{false};
PyObject* m_container{nullptr};
PyObject* m_key{nullptr};
public:
// Rvalue (e.g. x = ob[42];)
const Object operator[] (const Object& key) const {
return Object{ PyObject_GetItem( p, key.p ) };
}
// Don't know yet
Object operator[] (const Object& key) {
return Object{ *this, key };
}
// notice we set the m_resolve_me flag
// as we don't yet know L/Rvalue-ness
Object( const Object& c, const Object& k )
: m_container{c.p}, m_key{k.p}, m_resolve_me{true}
{
// for all but lvalue access (ob[idx]=...), ob[idx] will be valid
p = PyObject_GetItem( m_container, m_key );
if( p == nullptr ) {
// ... However in the case of lvalue access,
// PyObject_GetItem will set Python's error indicator
// so we must flush that error, as it was expected!
PyErr_Clear();
p = charge(Py_None);
}
// ^ either way, p ends up charged
}
public:
// this will attempt to convert ANY rhs to Object, which takes advantage of ALL the above constructor overrides
Object& operator=( const Object& rhs )
{
/*
1) normal situation
2) this object is m_resolve_me, and we are assigning
a normal object to it
3) this object is m_resolve_me, and we are assigning
a m_resolve_me object to it
4) this object is normal, and we are assigning a m_resolve_me object to it
1) we need to charge p
2) same
3) same
4) same
The only important thing is: we have to be neutral to rhs.p
That means we have to charge it, as we will be
subsequently neutralising it in the destructor
*/
if( &rhs != this )
*this = charge(rhs.p);
return *this;
}
// (Always) assume charged pointer
Object& operator=( PyObject* pyob )
{
if( m_resolve_me ) {
PyObject_SetItem( m_container, m_key, pyob );
m_resolve_me = false;
}
set_ptr( pyob );
return *this;
}