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Why is the following code, which used to compile fine some years ago,
not compiling anymore?

g++ test.cpp
test.cpp: In member function ‘int D<T>::Size()’:
test.cpp:35: error: ‘v’ was not declared in this scope
test.cpp:35: error: ‘k’ was not declared in this scope


// test.cpp

template <typename T = int>
class B
{
public:
int v, k;

B(int Av, int Ak)
{
v = Av;
k = Ak;
}

virtual int Size() = 0;

//...
};

template <typename T = int>
class D : public B<T>
{
public:
D(int Av, int Ak) : B<T>(Av, Ak)
{
}

int Size()
{
return v * k;
}
};

int main(int argc, char* argv[])
{
D<long> C(10, 2);
//...
return 0;
}

[REPOST]

Why is the following code, which used to compile fine some years ago,
not compiling anymore?

g++ test.cpp
test.cpp: In member function ‘int D<T>::Size()’:
test.cpp:35: error: ‘v’ was not declared in this scope
test.cpp:35: error: ‘k’ was not declared in this scope


// test.cpp

template <typename T = int>
class B
{
public:
int v, k;

B(int Av, int Ak)
{
v = Av;
k = Ak;
}

virtual int Size() = 0;

//...
};


template <typename T = int>
class D : public B<T>
{
public:
D(int Av, int Ak) : B<T>(Av, Ak)
{
}

int Size()
{
return v * k;
}
};


int main(int argc, char* argv[])
{
D<long> C(10, 2);
//...
return 0;
}

On 2010-12-04 09:40, Paavo Helde wrote:
> Student<> wrote in news:idcqa2$19o$1
> @speranza.aioe.org:
>
>> Why is the following code, which used to compile fine some years ago,
>> not compiling anymore?
>>
>> g++ test.cpp
>> test.cpp: In member function ?€?int D<T>::Size()?€™:
>> test.cpp:35: error: ?€?v?€™ was not declared in this scope
>> test.cpp:35: error: ?€?k?€™ was not declared in this scope
>>
>>
>> // test.cpp
>>
>> template<typename T = int>
>> class B
>> {
>> public:
>> int v, k;
>>
>> B(int Av, int Ak)
>> {
>> v = Av;
>> k = Ak;
>> }
>>
>> virtual int Size() = 0;
>>
>> //...
>> };
>>
>> template<typename T = int>
>> class D : public B<T>
>> {
>> public:
>> D(int Av, int Ak) : B<T>(Av, Ak)
>> {
>> }
>>
>> int Size()
>> {
>> return v * k;
>
> The base class depends on the template argument and can be different in
> different specializations, so the compiler cannot be sure what v and k
> really represent. Clarify with:
>
> return this->v * this->k;
>
>
> hth
> Paavo

Hey, cool, thanks, this solves the problem,
although I'm not really convinced why one has to use "this" there.

On 2010-12-04 09:54, Student wrote:
> On 2010-12-04 09:40, Paavo Helde wrote:
>> Student<> wrote in news:idcqa2$19o$1
>> @speranza.aioe.org:
>>
>>> int Size()
>>> {
>>> return v * k;
>>
>> The base class depends on the template argument and can be different in
>> different specializations, so the compiler cannot be sure what v and k
>> really represent. Clarify with:
>>
>> return this->v * this->k;
>>
>>
>> hth
>> Paavo
>
> Hey, cool, thanks, this solves the problem,
> although I'm not really convinced why one has to use "this" there.

Hmm... it's a nightmare to change all the code.
I think this is clearly a compiler bug.

Student <> writes:

> On 2010-12-04 09:54, Student wrote:
>> On 2010-12-04 09:40, Paavo Helde wrote:
>>> Student<> wrote in news:idcqa2$19o$1
>>> @speranza.aioe.org:
>>>
>>>> int Size()
>>>> {
>>>> return v * k;
>>>
>>> The base class depends on the template argument and can be different in
>>> different specializations, so the compiler cannot be sure what v and k
>>> really represent. Clarify with:
>>>
>>> return this->v * this->k;
>>>
>>>
>>> hth
>>> Paavo
>>
>> Hey, cool, thanks, this solves the problem,
>> although I'm not really convinced why one has to use "this" there.
>
> Hmm... it's a nightmare to change all the code.
> I think this is clearly a compiler bug.

No. See e.g. http://www.parashift.com/c++-faq-lite/templates.html
section

[35.19] Why am I getting errors when my template-derived-class uses a
member it inherits from its template-base-class?

for further explanation.

hth
--
m

Hi,

Paavo Helde wrote:
> Also, it seems you are trying to mix templates with OOP and
> virtual methods, this is not a very good design as they both solve
> similar problems, but in different ways which may easily conflict each
> other.

why do you think that these concepts should be used mutually exclusive?

I use templates to support interface implementations from time to time.
This will obviously mix virtual methods and templates.


Marcel

On 2010-12-04 10:29, Paavo Helde wrote:
> Student<> wrote in news:idd041$bue$1
> @speranza.aioe.org:
>
>> On 2010-12-04 09:54, Student wrote:
>>> On 2010-12-04 09:40, Paavo Helde wrote:
>>>> Student<> wrote in news:idcqa2$19o$1
>>>> @speranza.aioe.org:
>>>>
>>>>> int Size()
>>>>> {
>>>>> return v * k;
>>>>
>>>> The base class depends on the template argument and can be different
> in
>>>> different specializations, so the compiler cannot be sure what v and
> k
>>>> really represent. Clarify with:
>>>>
>>>> return this->v * this->k;
>>>>
>>
>> Hmm... it's a nightmare to change all the code.
>> I think this is clearly a compiler bug.
>
> No, it's not a bug, it's a feature Do not expect this to go away.
>
> Normally there are not so many templated base classes used in the code,
> why do you think you have to change "all the code"? Normally all member
> variables are private, so you would not use them in derived classes
> anyway. Also, it seems you are trying to mix templates with OOP and
> virtual methods, this is not a very good design as they both solve
> similar problems, but in different ways which may easily conflict each
> other. At least be prepared to having some complicated and cumbersome
> code if you insist on using them together.

Since this code compiled fine some years ago,
does anybody know if there is a compiler switch in g++
to get the code compile again without the above ugly "this->" 'workaround'?

g++ --version
g++ (Debian 4.4.4-11) 4.4.5 20100824 (prerelease)

Student wrote:

> Since this code compiled fine some years ago,
> does anybody know if there is a compiler switch in g++
> to get the code compile again without the above ugly "this->" 'workaround'?

Not that I know of. Is the solution of using "using" also too ugly for you?

On 2010-12-04 15:08, Marc wrote:
> Student wrote:
>
>> Since this code compiled fine some years ago,
>> does anybody know if there is a compiler switch in g++
>> to get the code compile again without the above ugly "this->" 'workaround'?
>
> Not that I know of. Is the solution of using "using" also too ugly for you?

To be honest: yes.
Because in the math formulae I use there's of course no such thing
as "this->", so then why should I cripple the code with it?
For me, the use of "this->" makes no sense at all.

On Dec 5, 8:31*pm, Student <inva...@amitrader.com> wrote:
> On 2010-12-04 15:08, Marc wrote:
>
> > Student *wrote:
>
> >> Since this code compiled fine some years ago,
> >> does anybody know if there is a compiler switch in g++
> >> to get the code compile again without the above ugly "this->" 'workaround'?
>
> > Not that I know of. Is the solution of using "using" also too ugly for you?
>
> To be honest: yes.
> Because in the math formulae I use there's of course no such thing
> as "this->", so then why should I cripple the code with it?
> For me, the use of "this->" makes no sense at all.

You perhaps did not understand the question there. It was what is ugly
about that solution:


// test.cpp
template <typename T = int>
class B
{
public:
int v, k;
B( int Av, int Ak )
: v( Av )
, k( Ak )
{}
virtual int Size() = 0;
//...
};

template <typename T = int>
class D : public B<T>
{
public:
using B<T>::v;
using B<T>::k;
D( int Av, int Ak )
: B<T>( Av, Ak )
{}
int Size()
{
return v * k;
}
};

int main(int argc, char* argv[])
{
D<long> C(10, 2);
//...
return 0;
}