数组订阅:返回引用与代理类方法 [英] Array Subscription: returning Reference vs proxy class method
问题描述
在寻找为模板类重载Subscript('[]')运算符的方法时,我遇到了两种不同的技术.
While searching for methods for overloading Subscript('[]') operator for template class, I came across two different techniques.
第一种技术:
直接将operator []
返回指针重载到容器,这将允许读取值和分配值.此技术的示例实现:
Overloading the operator []
returning pointer to the container directly, which will allow both reading value and assigning value. A sample implementation of this technique:
template <class T>
class X
{
int _size;
T *container;
public:
X(int sz)
{
_size=sz;
container=new T[sz]();
}
~X()
{
}
T& operator [](int indx)
{
return container[indx];
}
};
main()
为:
X<int> sample(100);
cout<<sample[9]<<endl;
sample[9]=9;
cout<<sample[9]<<endl;
输出:
0
9
第二种方法:
第二种技术涉及声明一个代理类,并通过该类重载operator =
.此技术的示例实现:
Second technique involves declaring a proxy class and overloading the operator =
via that class. A sample implementation of this technique:
template <class T>
class X
{
int _size;
T *container;
public:
X(int sz)
{
_size=sz;
container=new T[sz]();
}
~X()
{
}
class Proxy
{
int indx;
X<T> &parent;
public:
Proxy(X<T> &p, int x) : parent(p),indx(x)
{
}
void operator =(T assgn)
{
parent.container[indx]=assgn;
}
operator T const &()
{
return parent.container[indx];
}
friend class X<T>;//unnecessary line, I know!
};
Proxy operator[](int indx)
{
return Proxy(*this,indx);
}
};
使用相同的main()
,我们将获得相同的输出.
With same main()
we get same output.
我个人喜欢第二种方法.但是,我真的很想比较这两种方法.这两种技术的主要功能区别是什么?这些方法各自有什么优势?
I personally like the second method. But, I really want to compare these two methods. What are the major functional difference of these two techniques. What advantages each of these method have?
推荐答案
如果您要公开未按原样存储的元素序列(需要从和进行转换,则可以使用您描述的基于代理的技术)存储空间),或者不能简单地通过引用进行访问.一个示例是std :: vector< bool>:在存储的每个字节(每个位一个)中打包八个bool.这样存储它们,就不可能返回对单个此类布尔的引用,因此索引运算符将返回代理对象"来支持对所包含布尔的读取和写入.
The proxy-based technique you describe can be used if you want to expose a sequence of elements that are not stored as-such (requiring a conversion from and to storage) or that cannot simply be accessed by-reference. An example is std::vector < bool >: it packs eight bools in every byte (one in each bit) in storage. Storing them that way, it is not possible to return a reference to a single such bool, so the index operator returns a "proxy-object" instead to support reading and writing of the contained bools.
如果可以返回对存储对象的直接引用,则除非希望限制分配(例如仅允许在容器中使用正值),否则将其包装在代理中并没有真正的优势. ).
If you can return a direct reference to the stored objects, there is no real advantage to wrapping it in a proxy unless you want to restrict the assignment (only allow positive values in the container for example).
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