Which platforms don't use string comparison in type_info op==?

Question

Here is a typical implementation of type_info::operator==:

#if _PLATFORM_SUPPORTS_UNIQUE_TYPEINFO
    bool operator==(const type_info& __rhs) const {
      return __mangled_name == __rhs.__mangled_name;
    }
#else
    bool operator==(const type_info& __rhs) const {
      return __mangled_name == __rhs.__mangled_name ||
             strcmp(__mangled_name, __rhs.__mangled_name) == 0;
    }
#endif

In libstdc++ it's controlled with __GXX_MERGED_TYPEINFO_NAMES,
in libc++ it's _LIBCPP_NONUNIQUE_RTTI_BIT,
MSVC always compares strings.

What are the platforms which don't compare strings?

Maxim Egorushkin · Accepted Answer · 2018-12-24T18:24:11.280

4

In libstdc++ it's controlled with __GXX_MERGED_TYPEINFO_NAMES

In newer versions of gcc (since 23 Jul 2009) this macro is set to 0 by default. It always compares pointers first and if that fails they do the full string comparison. See here:

We used to do inline pointer comparison by default if weak symbols are available, but even with weak symbols sometimes names are not merged when objects are loaded with RTLD_LOCAL, so now we always use strcmp by default.

edited Dec 24 '18 at 18:24

answered Dec 22 '18 at 17:42

Maxim Egorushkin

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2

Notably, it's *possible* for a target to override this, but no target actually does. – o11c Dec 24 '18 at 19:02
1

@o11c 3 targets do override it: `__GXX_MERGED_TYPEINFO_NAMES=0`. – Maxim Egorushkin Dec 24 '18 at 19:08
1

Er, right, but they overwrite it to the value it already has. – o11c Dec 24 '18 at 19:56

Which platforms don't use string comparison in type_info op==?

1 Answers1

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