C++ template class behaves differently for different compilers

Question

I am working with templates in C++. Is there any difference in using templates and friend class when compiled with MSVC compiler and when using Mingw gcc compiler. My code successfully compiles and gives the desired output when compiled with MSVC but it gives error when compiled with gcc. Below is my code,

///////////Record.h/////////////////////
#include "Base.h"

class Derived1;
class Derived2;
template <class TYPE_LIST> class List;

class FRecord
{
public:
    FRecord();
    virtual ~FRecord();

    friend class Base;
#if _MSC_VER <= 1200
    friend class List<Derived1>;
    friend class List<Derived2>;
#else
    template <class TYPE_LIST> friend class List;
#endif
};

///////////////////////////////////////////////////////////////

///////////////////Base.h/////////////////////////////////

class Base
{
public:
    Base(const HEADER *hc, const FRecord *fr);
    virtual ~Base();    
    __inline bool IsNonValid() const;

protected:
    quint32 Size;
};

/////////////////////////////////////
// Data
/////////////////////////////////////
template <class TYPE_LIST>
class Data : public TYPE_LIST
{
public:
    Data(const HEADER *hc, const FRecord *fr) : TYPE_LIST(hc, fr)
    {
        QString val = IsNonValid() ? "Non" : "";
        LOG0("Data ("<<  val << " Valid)");
    }

    virtual ~Data()
    {
        LOG0("Data deleted");
    }
};  // Data

///////////////////////////////////////////////////////////////////////////////////////

When compiled the above code with MSVC gives desired output but when compiled with Mingw GCC compiler it gives following error,

Base.h:1154: error: there are no arguments to 'IsNonValid' that depend on a template parameter, so a declaration of 'IsNonValid' must be available
Base.h:1553: error: 'Size' was not declared in this scope

What could be the possible solution to this problem? Thanks in advance.

score 11 · Accepted Answer · answered Oct 11 '13 at 13:04

MSVC does not implement two-phase name lookup correctly. GCC is correct in reporting this error.

The cause is that names used inside a template which do not depend on the template's parameters are (should be in the case of VC) looked up when the template is defined, not when it's instantiated.

In your case, the compiler has no way of telling that IsNonValid will come from the base class, so it rightfully complains it doesn't know it. There are two possible solutions:

Qualify the access to IsNonValid, so that it's clear to the compiler it (potentially) depends on the template parameters:
```
QString val = this->IsNonValid() ? "Non" : "";

// or

QString val = TYPE_LIST::IsNonValid() ? "Non" : "";
```

Introduce the inherited name into the scope of the derived class:

template <class TYPE_LIST>
class Data : public TYPE_LIST
{
public:
  using TYPE_LIST::IsNonValid;
  // the rest as you had it originally

Either of these will make the name dependent and thus postpone its lookup until instnatiation, when the value of TYPE_LIST is actually known.

Thanks @Angew: Your solution solved my problem.:)..Now how can I changed the status of my question to SOLVED? — user2765235, Oct 12 '13 at 11:29
@user2765235 By accepting the answer which you consider solved the problem. You do this by clicking the green tick-mark next to the answer. See the [help](http://stackoverflow.com/help/accepted-answer) for more info. — Angew is no longer proud of SO, Oct 13 '13 at 09:19

score 7 · Answer 2 · edited Oct 11 '13 at 14:20

7

gcc is correct. You need to add this-> to delay lookup until instantiation time.

this->IsNonValid();

MSVC is non-conforming in that it delays all lookups until instantiation time as it doesn't properly implement two-phase name lookup.

edited Oct 11 '13 at 14:20

David Rodríguez - dribeas

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answered Oct 11 '13 at 13:02

Jesse Good

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C++ template class behaves differently for different compilers

2 Answers2