Are implementations allowed to add public members to standard types?

Question

Are C++ standard library implementations allowed to add public (and protected) members to standard types' interfaces? N3797 17.6.5.5 [member.functions]/2 says:

An implementation may declare additional non-virtual member function signatures within a class:

— by adding arguments with default values to a member function signature; [ Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note ]

— by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and

— by adding a member function signature for a member function name.

Does this mean that a standard library cannot add any additional public members with names not mentioned in the standard under any circumstances (that include, for example, reserved identifiers)?

A tiny bit of explanation: this is the text about adding signatures (which I assume talks about new signatures just for functions that are already defined to be there, so no new names) I managed to find in the standard. There is also the footnote 189, which says:

A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.

All this text originates from [member.functions], so it is clearly about member functions only. My question is more generic and asks for any references I could've missed: is a standard library implementation allowed to add new names to public (and/or protected) interfaces of a standard type, be it data or function members?

Answer 1

I believe you have what you need with a combination of foot note 189 which says:

A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.

and section 17.6.5.11 Derived classes which says:

An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.

but does not add any restrictions, i.e. it does not let's say restrict the access qualifiers etc...

and we can see libstdc++ uses derived classes pretty effectively, for example in stl_vector.h. Although as far as I can see libstdc++ does seem to eschew adding public data members but that is probably more for clean design.

At minimum, this looks under-specified but if you stick to something similar to libstdc++ implementation style you should be good.

Answer 2

I think that the key to reading footnote 189 is the phrase would otherwise not be called by a valid C++ program.

Remember that identifiers beginning with an underscore followed by a capital letter (or containing two consecutive underscores anywhere) are reserved for the implementation. (section 17.6.4.3.2)

So implementations are free to add public/protected member functions that are named in that manner.

For example, in libc++, std::vector has a protected member function named __throw_length_error