Problem with global namespace on g++ 4.8.5 (maybe incomplete C++11 support)

Question

I have prepared a project on an up-to-date computer using -std=c++11. Now, I have to compile it on g++ 4.8.5 (Red Hat 4.8.5-39). To my surprise, support for C++11 standard on this system seems to be incomplete. Please note, that the project depends on gsoap library which is also older on the production system.

I am using the following construct:

class GeoGrid : public ::GeoGrid::GeoGridServiceSoapBindingProxy
{
public:
  ...
  using ContextType = ::GeoGrid::GeoGrid2__ContextType;
  ...
}

I get the following error on the old system:

api-2-0.h:434:23: error: expected type-specifier before ‘::’ token
   using ContextType = ::GeoGrid::GeoGrid2__ContextType;

I do not know if this is produced due to the old gsoap or just old g++.

My question is if this can be resolved without upgrading the system? I have to admit that I do not completelly understand all the consequences of using ‘::’ token at the beginning, it was simply a lucky guess that works perfectly on the new system.

Why do you think that the error message implies a lack of C++11 support? According to this page, GCC 4.8 supports practically all C++11 (core language) features. — Daniel Langr
– Daniel Langr, Commented Oct 6, 2020 at 17:37
so, do you think, that gobal namespace is perfectly and identically suported on both systems - then I have to look for the error elsewhere, probably different behaviour of gsoap — meolic
– meolic, Commented Oct 6, 2020 at 17:50
To rule C++11 support out as a concern, maybe you can try using a regular typedef? — Fatih BAKIR
– Fatih BAKIR, Commented Oct 6, 2020 at 18:56

Swift - Friday Pie · Accepted Answer · 2020-10-07 08:21:18Z

Your snippet consistent with following situation:

/*
struct A {
    struct B {};
}; */

namespace X 
{
struct A {
    using B = ::A::B;    
};
}

producing with new gcc (v10)

error: 'A' in namespace '::' does not name a type
     using C = ::A::B;

or this with old versions (v 4.7+)

 error: expected type-specifier before '::' token
     using C = ::A::B;

The error above is generated only in the case when there is no A::B (GeoGrid::GeoGridServiceSoapBindingProxy in your case) in global namespace, thus ::A::B is not a legal typename, therefore compiler considers A being a legal typename from current context's scope and messages about lack of required identifier of context before the scope divider ::. The error message for newer versions is more readable than the technically correct but misleading older one.

Another version of this situation would be

struct A {
    using C = ::A::B;
    struct B {};
    
};

At the line where using C is present, B doesn't exist yet as a type, resulting in same error message for GCC 4.8. Newer versions would tell that type is incomplete.

 error: invalid use of incomplete type 'struct A'
     using C = ::A::B;

In both cases it's an indication of malformed program, not of lack of standard support.

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