The C++ standard library reserves the following kinds of names:
names with external linkage
If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by this Clause, its behavior is undefined.
In namespace std, the following names are reserved for previous standardization:
A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in [dcl.attr].
Each name declared as an object with external linkage in a header is reserved to the implementation to designate that library object with external linkage,177 both in namespace std and in the global namespace.
Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.178
Each name from the C standard library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.
Each function signature from the C standard library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage,179 or as a name of namespace scope in the global namespace.
The list of such reserved names includes errno, declared or defined in <cerrno>.
The list of such reserved function signatures with external linkage includes setjmp(jmp_buf), declared or defined in <csetjmp>, and va_end(va_list), declared or defined in <cstdarg>.
The function signatures declared in <cuchar>, <cwchar>, and <cwctype> are always reserved, notwithstanding the restrictions imposed in subclause 4.5.1 of Amendment 1 to the C Standard for these headers.
For each type T from the C standard library,180 the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.
These types are clock_t, div_t, FILE, fpos_t, lconv, ldiv_t, mbstate_t, ptrdiff_t, sig_atomic_t, size_t, time_t, tm, va_list, wctrans_t, wctype_t, and wint_t.
Literal suffix identifiers that do not start with an underscore are reserved for future standardization.