What is the correct template type?

JiiPee

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Dec 27, 2014, 5:36:18 PM12/27/14

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I am trying to make xx (in pair2_derived) to be a reference member
variable so that it refers to "first" in pair2. What should I put as a
type for xx? I first tried T (because it is/will be the type in real
life), but I get "error: expected ';' at end of member declaration". I
tried other ones as well like pair2<T, T>::_T1 but none worked. I am not
very familiar with templates so would need some help here, thanks.

template<class _T1, class _T2>
struct pair2
{
_T1 first;
_T2 second;
};

template <typename T>
class pair2_derived: public pair2<T, T>
{
public:
T& xx = pair2<T, T>::first; // xx has a wrong type...what should the
type of xx be?
};

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Paavo Helde

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Dec 27, 2014, 6:17:42 PM12/27/14

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JiiPee <n...@notvalid.com> wrote in news:svGnw.903302$u67.8...@fx08.am4:

> I am trying to make xx (in pair2_derived) to be a reference member
> variable so that it refers to "first" in pair2. What should I put as a
> type for xx? I first tried T (because it is/will be the type in real
> life), but I get "error: expected ';' at end of member declaration". I
> tried other ones as well like pair2<T, T>::_T1 but none worked. I am
not
> very familiar with templates so would need some help here, thanks.
>
> template<class _T1, class _T2>
> struct pair2
> {
> _T1 first;
> _T2 second;
> };

Identifiers like _T1 are reserved for implementations, it is illegal to
use them in your own code.

> template <typename T>
> class pair2_derived: public pair2<T, T>
> {
> public:
> T& xx = pair2<T, T>::first; // xx has a wrong type...what should
the
> type of xx be?
> };

template <typename T>
class pair2_derived: public pair2<T, T>
{
public:

T& xx;
pair2_derived(): xx(this->first) {}
};

JiiPee

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Dec 27, 2014, 6:33:38 PM12/27/14

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On 27/12/2014 23:17, Paavo Helde wrote:
> JiiPee <n...@notvalid.com> wrote in news:svGnw.903302$u67.8...@fx08.am4:
>
>> I am trying to make xx (in pair2_derived) to be a reference member
>> variable so that it refers to "first" in pair2. What should I put as a
>> type for xx? I first tried T (because it is/will be the type in real
>> life), but I get "error: expected ';' at end of member declaration". I
>> tried other ones as well like pair2<T, T>::_T1 but none worked. I am
> not
>> very familiar with templates so would need some help here, thanks.
>>
>> template<class _T1, class _T2>
>> struct pair2
>> {
>> _T1 first;
>> _T2 second;
>> };
> Identifiers like _T1 are reserved for implementations, it is illegal to
> use them in your own code.
>
>

Ok, but that does not seem to be the problem here. Even if I take _ off
from it for example
pair2<T, T>::T1& xx = pair2<T, T>::first;

does not work.

Öö Tiib

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Dec 27, 2014, 6:34:57 PM12/27/14

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On Sunday, December 28, 2014 12:36:18 AM UTC+2, JiiPee wrote:
> I am trying to make xx (in pair2_derived) to be a reference member
> variable so that it refers to "first" in pair2. What should I put as a
> type for xx? I first tried T (because it is/will be the type in real
> life), but I get "error: expected ';' at end of member declaration". I
> tried other ones as well like pair2<T, T>::_T1 but none worked. I am not
> very familiar with templates so would need some help here, thanks.
>
> template<class _T1, class _T2>
> struct pair2
> {
> _T1 first;
> _T2 second;
> };
>
> template <typename T>
> class pair2_derived: public pair2<T, T>
> {
> public:
> T& xx = pair2<T, T>::first; // xx has a wrong type...what should the
> type of xx be?
> };

No compiler nearby but it might be that this works like you want:

template <typename T>
class pair2_derived: public pair2<T, T>
{
public:

typedef pair2<T, T> base;
T& xx = base::first;
};

int main()
{
pair2_derived<int> x;
x.first = 42;
x.second = 43;
std::cout << "xx is:" << x.xx << std::endl;
return 0;
}

JiiPee

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Dec 27, 2014, 6:45:36 PM12/27/14

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wow. That's it! Thanks. Now I need to google why its like this :). good
to learn ....
never heard about "base"...but I ll google.

JiiPee

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Dec 27, 2014, 6:48:08 PM12/27/14

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sorry, its not about "base", its the trick typedef.... obviously it can
be like base2

JiiPee

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Dec 27, 2014, 6:57:13 PM12/27/14

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On 27/12/2014 23:34, Öö Tiib wrote:

> No compiler nearby but it might be that this works like you want:
> template <typename T> class pair2_derived: public pair2<T, T> {
> public: typedef pair2<T, T> base; T& xx = base::first; }; int main() {
> pair2_derived<int> x; x.first = 42; x.second = 43; std::cout << "xx
> is:" << x.xx << std::endl; return 0; }

I need this because I am trying to use std::pair as a base for 2dvector
and 2dsize. Just testing how it would go... so I have to put
first/second to be refered by x/y and height/width.

Öö Tiib

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Dec 27, 2014, 7:58:42 PM12/27/14

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So the goal is renaming/aliases? Other question is why you need pair there?
It does not have much of functionality. Just relational operators and
'swap'.

The references are bad for renaming/aliasing because these take some memory.
Your derived objects will be bigger. It is better to use short accessory
functions for such aliases. Sort of like:

struct Vertex
{
float first() const { return values[0]; }
float second() const { return values[1]; }
float third() const { return values[2]; }

float x() const { return values[0]; }
float y() const { return values[1]; }
float z() const { return values[2]; }

float operator [] (int i) const { return values[i]; }
float& operator [] (int i) { return values[i]; }

float[3] values;
};

The short member function calls will be usually optimized out by
compiler.

JiiPee

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Dec 27, 2014, 8:05:14 PM12/27/14

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so like get/set functions. I have been thinking about them, but I do not
like x() because its so long. only writing "x" is shorter. I would like
to do: obj.x rather than obj.x() . but ye there is that tradeoff. So to
fix your point I would need to not use pair but rather have x/y inside
the class, T x; .

most graphical libraries do x, so I would prefer that here... it is used
so much in the code also so it needs to be as short as possible I think.

JiiPee

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Dec 27, 2014, 8:12:11 PM12/27/14

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On 28/12/2014 00:58, Öö Tiib wrote:

> On Sunday, December 28, 2014 1:57:13 AM UTC+2, JiiPee wrote:
>> On 27/12/2014 23:34, Öö Tiib wrote:
>>> No compiler nearby but it might be that this works like you want:
>>> template <typename T> class pair2_derived: public pair2<T, T> {
>>> public: typedef pair2<T, T> base; T& xx = base::first; }; int main() {
>>> pair2_derived<int> x; x.first = 42; x.second = 43; std::cout << "xx
>>> is:" << x.xx << std::endl; return 0; }
>> I need this because I am trying to use std::pair as a base for 2dvector
>> and 2dsize. Just testing how it would go... so I have to put
>> first/second to be refered by x/y and height/width.
> So the goal is renaming/aliases? Other question is why you need pair there?
> It does not have much of functionality. Just relational operators and
> 'swap'.
>
> The references are bad for renaming/aliasing because these take some memory.
> Your derived objects will be bigger. It is better to use short accessory
> functions for such aliases. Sort of like:
>
>

oh. was thinking/hoping the compiler would optimize that away... why
would the compiler not be able to do that?