Can I use the iterator Libraries' Access Functions on Nonstandard Containers?

问题

The iterator library has been introducing a lot of access functions over the course of C++11, C++14, and C++17:

• begin/end
• cbegin/cend
• crbegin/crend
• data
• empty
• rbegin/rend
• size

Can I use these on any container, even nonstandard containers (provided they supply an accessible corresponding method?) For example given a QVector foo can I do this:

const auto bar = begin(foo);

回答1:

The declarations for std::begin are as follow (from §24.7):

template <class C> auto begin(C& c) -> decltype(c.begin());
template <class C> auto begin(const C& c) -> decltype(c.begin());

So these functions will be defined for any class C such that c.begin() is "valid" (does exist). The standard also guarantees that these will:

Returns: c.begin().

So yes you can use begin(c) on any container of type C as long as either:

• The member function C::begin() is provided.
• There exist a begin(C const&) or begin(C &) function.

The standalone begin function should not be in the std:: namespace but rather in the same namespace as your class C so name-lookup can find it.

回答2:

As I understand it, your question seem to be: if the container provides a member function begin, can I then call it as a free function? The answer to your question is yes, because a templated free function begin is provided by the standard, that simply tries to call the member function begin; from http://en.cppreference.com/w/cpp/iterator/begin: "Returns an iterator to the beginning of the given container c or array array. These templates rely on C::begin() having a reasonable implementation.".

来源：https://stackoverflow.com/questions/38101301/can-i-use-the-iterator-libraries-access-functions-on-nonstandard-containers