/PURE_CNAME
       /PURE_CNAME (D) (/STANDARD=STRICT_ANSI)
    /NOPURE_CNAME (D) (All other modes)

    Affects insertion of the names into the global namespace by
    <cname> headers.

    In /PURE_CNAME mode, the <cname> headers insert the names into
    the std namespace only, as defined by the C++ Standard. In this
    mode, the __PURE_CNAME and __HIDE_FORBIDDEN_NAMES  macros are
    predefined by the compiler.

    In /NOPURE_CNAME mode, the <cname> headers insert the name into
    the std namespace and also into the global namespace. In this
    mode, the __PURE_CNAME and __HIDE_FORBIDDEN_NAMES  macros are not
    predefined by the compiler.

    The default depends on the standard mode:

    o  In /STANDARD=STRICT_ANSI mode, the default is /PURE_CNAME.

    o  In all other standard modes, the default is /NOPURE_CNAME.

    Inclusion of a <name> header instead of its <cname> counterpart
    (for example, <stdio.h> instead of <cstdio>) results in inserting
    names defined in the header into both the std namespace and the
    global namespace. Effectively, this is the same as the inclusion
    of a <cname> header in /NOPURE_CNAME mode.