重载（c）开始/（c）结束 [英] Overloading (c)begin/(c)end

问题描述

我试图为类重载（c）begin / （c）end 函数，以便能够调用C ++ 11基于范围的for循环。

I tried to overload (c)begin/(c)end functions for a class so as to be able to call C++11 range-based for loop.

在大多数情况下都可以使用，但是我无法理解和解决一个问题：

It works in most of the cases, but I don't manage to understand and solve one :

for (auto const& point : fProjectData->getPoints()){ ... }

此行返回错误：

Error C2662: 'MyCollection<T>::begin' : cannot convert 'this' pointer from 'const MyCollection' to 'MyCollection<T> &'

，因为 fProjectData 是指针到const。如果我将其设为非常量，则它确实可以工作。考虑到 cbegin（）& cend（）的精确度与 begin（）& end（）函数。

because fProjectData is a pointer to const. If I make it non-const, it does work. I don't understand why, considering that cbegin() & cend() are developped with exactness as begin() & end() functions.

这是我在MyCollection中开发的函数（在头文件中）：

Here are my functions developped (in the header file) in MyCollection:

/// \returns the begin iterator
typename std::list<T>::iterator begin() {
    return objects.begin();
}

/// \returns the begin const iterator
typename std::list<T>::const_iterator cbegin() const {
    return objects.cbegin();
}

/// \returns the end iterator
typename std::list<T>::iterator end() {
    return objects.end();
}

/// \returns the end const iterator
typename std::list<T>::const_iterator cend() const {
    return objects.cend();
}

有任何想法吗？

推荐答案

基于范围的for循环（用于类类型范围）查找开始和结束功能。 开始和 cend 完全不考虑：

A range-based for loop (for a class-type range) looks up for begin and end functions. cbegin and cend are not considered at all:

§6.5.4 [stmt.ranged] / p1 *：

§ 6.5.4 [stmt.ranged]/p1 *:

[...]

[...]

• 如果 _RangeT 是类类型，则 unqualified-ids 开始和<在类的范围内查找code> end ，就像按类成员访问查找（3.4.5）一样，并且如果（或两者）找到至少一个声明， begin-expr 和 end-expr 为 __ range.begin（）和 __ range.end（）；

• if _RangeT is a class type, the unqualified-ids begin and end are looked up in the scope of class _RangeT as if by class member access lookup (3.4.5), and if either (or both) finds at least one declaration, begin-expr and end-expr are __range.begin() and __range.end(), respectively;

否则， begin-expr 和 end-expr 是开始（__range）和 end（__ range），分别在关联的命名空间（3.4.2）中查找开始和 end 。 [注意：不执行普通的不合格查询（3.4.1）。 — 尾注]

otherwise, begin-expr and end-expr are begin(__range) and end(__range), respectively, where begin and end are looked up in the associated namespaces (3.4.2). [ Note: Ordinary unqualified lookup (3.4.1) is not performed. — end note ]

对于 const 限定的范围，相关的成员函数也必须是 const 限定的（或应与<$ c $一起调用c> const 限定的实例（如果使用后一个选项）。您需要引入其他重载：

For a const-qualified range the related member functions must be const-qualified as well (or should be callable with a const-qualified instance if the latter option is in use). You'd need to introduce additional overloads:

typename std::list<T>::iterator begin() {
    return objects.begin();
}

typename std::list<T>::const_iterator begin() const {
//                                            ~~~~^
    return objects.begin();
}

typename std::list<T>::const_iterator cbegin() const {
    return begin();
}

typename std::list<T>::iterator end() {
    return objects.end();
}

typename std::list<T>::const_iterator end() const {
//                                          ~~~~^
    return objects.end();
}

typename std::list<T>::const_iterator cend() const {
    return end();
}

演示

_{^* 措辞来自C ++ 14，但区别与所陈述的问题无关}

_{^* the wording comes from C++14, but the differences are unrelated to the problem as it is stated}

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