Overloading (c)begin/(c)end

问题

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.

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

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

This line returns error:

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

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.

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();
}

Any ideas?

回答1:

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 *:

[...]

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;

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 ]

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();
}

DEMO

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

来源：https://stackoverflow.com/questions/31581880/overloading-cbegin-cend

标签

c++

c++11

for-loop

const