自定义容器应该有免费的开始/结束功能吗?

A second alternative is to encapsulate the using-declarations of the previous solution into a separate adl namespace by providing non-member function templates adl_begin() and adl_end(), which can then also be found through ADL. Example code (based on comments by @Yakk):

// LegacyContainerBeginEnd.h 
// as before...

// ADLBeginEnd.h
namespace adl {

using std::begin; // <-- here, because otherwise decltype() will not find it 

template<class C> 
auto adl_begin(C && c) -> decltype(begin(std::forward<C>(c)))
{ 
    // using std::begin; // in C++14 this might work because decltype() is no longer needed
    return begin(std::forward<C>(c)); // try to find non-member, fall back on std::
}

// similary for cbegin(), end(), cend(), etc.

} // namespace adl

using adl::adl_begin; // will be visible in any compilation unit that includes this header

// print.h
# include "ADLBeginEnd.h" // brings adl_begin() and adl_end() into scope

template<class C>
void print(C const& c)
{
    // works for Standard Containers, C-style arrays and legacy Containers
    std::copy(adl_begin(c), adl_end(c), std::ostream_iterator<decltype(*adl_begin(c))>(std::cout, " ")); std::cout << "
";

    // alternative: also works for Standard Containers, C-style arrays and legacy Containers
    // does not need adl_begin() / adl_end(), but continues to work
    for (auto elem: c) std::cout << elem << " "; std::cout << "
";
}

优点:一致的调用约定，完全通用

// LegacyIntContainerBeginEnd.h
namespace std {

// retro-fitting begin() / end() interface on legacy IntContainer class 
// with incompatible names         
template<> 
auto begin(legacy::IntContainer& c) -> decltype(c.legacy_begin())
{ 
    return c.legacy_begin(); 
}

// similary for begin() taking const&, cbegin(), end(), cend(), etc.

} // namespace std

// LegacyContainer.h
namespace legacy {

template<class T>
class Container
{
public:
    // YES, DOCUMENT REALLY WELL THAT THE EXISTING CODE IS BEING MODIFIED
    auto begin() -> decltype(legacy_begin()) { return legacy_begin(); }
    auto end() -> decltype(legacy_end()) { return legacy_end(); }

    // rest of existing interface
};

} // namespace legacy

// print.h
template<class C>
void print(C const& c)
{
    // works for Standard Containers, C-style arrays as well as 
    // legacy::IntContainer and legacy::Container<T>
    std::copy(std::begin(c), std::end(c), std::ostream_iterator<decltype(*std::begin(c))>(std::cout, " ")); std::cout << "
";

    // alternative: also works for Standard Containers, C-style arrays and
    // legacy::IntContainer and legacy::Container<T>
    for (auto elem: c) std::cout << elem << " "; std::cout << "
";
}