C++/WinRT naming conventions

C++/WinRT has established the following naming conventions:

• The winrt::impl namespace is reserved for C++/WinRT, and you shouldn't use it in your application.
• In the winrt namespace, names that begin with a lowercase letter belong to C++/WinRT, but you may use them in your application. The documentation calls out those names that you can overload or specialize. For example, your application is permitted to specialize the winrt::is_guid_of function template.
• In sub-namespaces of the winrt namespace (except for winrt::impl), names that begin with an uppercase letter are available to your application.
• In all namespaces, names beginning with WINRT_IMPL_ are reserved for C++/WinRT, and you shouldn't use them in your application.
• In all namespaces, names beginning with WINRT_ (except those that begin with WINRT_IMPL_) are reserved for C++/WinRT. You may use them, and the documentation calls out those names that may be defined by your application, such as WINRT_LEAN_AND_MEAN.

It's common for applications to perform namespace composition, and import sub-namespaces of the winrt root namespace into the winrt root namespace:

namespace winrt
{
    using namespace winrt::Windows::Foundation;
}

Therefore, your application should adhere to the naming conventions above in sub-namespaces of the winrt namespace.

Here's a summary.

Namespace	Name	Apps may define	Apps may use
winrt::impl	Any	No	No
winrt and sub-namespaces (except impl)	Starts with lowercase letter	No	Yes
winrt and sub-namespaces (except impl)	Starts with uppercase letter	Yes	Yes
Any	WINRT_IMPL_*	No	No
Any	WINRT_* (except WINRT_IMPL_*)	Case-by-case	Yes