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Re: Template argument name resolution and compiler discrepancies

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On 2015-09-12 19:24, Jonathan Jones wrote:

>
> Hi folks,
>
> Look at the following example code:
>
>      template<typename>
>      class Base
>      {
>          typedef int ValueType;
>      };
>
>      template<typename ValueType>
>      struct Derived : public Base<ValueType>
>      {
>          Derived(ValueType);
>      };
>
>      Derived<float> obj(1);
>
> This code compiles cleanly with GCC 4.7.2 and Clang 3.5 (Xcode 6.2).
> However, it fails to compile with Visual Studio 2013, complaining that it
> cannot access private typedef "Base<ValueType>::ValueType".
>
> Furthermore, if you make the private typedef public:
>
>      template<typename>
>      class Base
>      {
>      public:
>          typedef int ValueType;
>      };
>
> It compiles on all three compilers, but the produces different answers.
> GCC and clang result in ValueType=float in the Derived constructor
> definition, whereas Visual Studio 2013 results in ValueType=int in the
> Derived constructor definition.
>
> What does the standard say should happen in these cases?  Which compiler(s)
> are closer to implementing the standard behavior?
>
> The standard is clear (see C++11 section 14.6.1 paragraph 9) about what
> happens when "Base" is not a template, but it seems more vague about when
> "Base" is a template (which the example above seems to demonstrate).
>
>
I would use different names when Base::ValueType is not the same as
Derived::ValueType.  :-)


Otherwise, I suspect this is one symptom of VC++ failing to do a proper
2-phase name lookup. When Base is a template, its ValueType should not be
visible in the declaration of Derived(ValueType). If VC++ postpones the
name lookup until the template is instantiated, it WILL be visible at that
point.

However, as you say, when Base is not a template its ValueType will shadow
the template parameter in Derived. That's one reason for using a different
name.


Bo Persson


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Template argument name resolution and compiler discrepancies Jonathan Jones <jhjones1969@googlemail.com> - 2015-09-12 11:24 -0600
  Re: Template argument name resolution and compiler discrepancies Bo Persson <bop@gmb.dk> - 2015-09-13 07:51 -0600

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