Are implementations allowed to add public members to standard types?
c++, c++11, c++14, language-lawyer
Solution
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.
Problem
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?