Pointer to member function - type

[jrx]

I've got my_error exception type, defined as:

class my_error : public std::runtime_er ror {
public:
my_error(const std::string& msg) : std::runtime_er ror(msg) {}
};

Then, I've got Wrapper class, that wraps my classes. I create it:

Wrapper<my_erro rmy_error_wrapp er;
my_error_wrappe r.addMethod(&my _error::what);

The template method addMethod looks like:

template <class T>
class Wrapper {
public:
[...]
template <class Result>
void addMethod(Resul t (T::*method)()) {
}
};

The problem is, that.. it doesn't work. It appears that, my_error::what
has the type
const char* (std::runtime_e rror::*)() const
not:
const char* (my_error::*)() const

Quite tricky, because std::runtime_er ror also derives what() from
std::exception.

Is there any way to give pointer to such method as an argument?
Except form:

template <class Result, class Base>
void addMethod(Resul t (Base::*method) ()) {
STATIC_ASSERT(i s_derived<Base, T>::value);
}

Another question:
Is it possible to obtain method type in some way like:
template <class Method>
void addMethod(Metho d m_ptr) {
}

I know there is something like "function type" used e.g. in
boost::signals to write:
boost::signal<v oid (const char*)my_signal ;
Is there something like that for methods?

thanks in advance ;)

JRX
--
JRX --dev \dot jrx \at gmail \dot com
If the words "open source" get you more excited than
the words "free porn"...you might be a Game Developer.
Registered Linux user# 383163

[kwikius]

Re: Pointer to member function - type

jrx wrote:

The problem is, that.. it doesn't work. It appears that, my_error::what
has the type
const char* (std::runtime_e rror::*)() const
not:
const char* (my_error::*)() const
>
Quite tricky, because std::runtime_er ror also derives what() from
std::exception.
>
Is there any way to give pointer to such method as an argument?

In the above I think you need to make the signature const e.g:

template <class T>
class Wrapper {
/*...*/
//############### ############### #############
void addMethod(Resul t (T::*method)() const) {
//############### ############### ##############
}
};

Unfortunately the error message is misleading. Try the above and the
function should be found in the base class.


To call it I think boost::function etc won't help. You could define
your own functor maybe like this:

#include <stdexcept>
#include <iostream>


template <typename Class>
struct what_functor{
Class* m_p_class;
const char* (Class::* m_pf)()const;
what_functor(Cl ass * p_class_in, const char* (Class::*
pf_in)()const)
: m_p_class(p_cla ss_in),m_pf(pf_ in){}

const char* operator()()
{
return ( m_p_class->*(m_pf))();
}
};

class my_error : public std::runtime_er ror {
public:
my_error(const std::string& msg) : std::runtime_er ror(msg) {}

};


int main()
{
my_error e("something" );
// need a class instance pointer and the member function pointer
// to init the functor
what_functor<my _errorfunc(&e,& my_error::what) ;

std::cout << func() <<'\n';
}

regards
Andy Little

[jrx]

Re: Pointer to member function - type

kwikius wrote:

>
In the above I think you need to make the signature const e.g:
>
template <class T>
class Wrapper {
/*...*/
//############### ############### #############
void addMethod(Resul t (T::*method)() const) {
//############### ############### ##############
}
};
>
Unfortunately the error message is misleading. Try the above and the
function should be found in the base class.

Unfortunately it doesn't work either.
Such signature:

template <class Return>
void addMethod(Retur n (T::*method)()c onst ) {
}

Causes error:
error: no matching function for call to
‘Wrapper<my_e rror>::addMetho d(const char*std::runti me_error::*)()c onst)’

>
>
To call it I think boost::function etc won't help. You could define
your own functor maybe like this:
>
#include <stdexcept>
#include <iostream>
>
>
template <typename Class>
struct what_functor{
Class* m_p_class;
const char* (Class::* m_pf)()const;
what_functor(Cl ass * p_class_in, const char* (Class::*
pf_in)()const)
: m_p_class(p_cla ss_in),m_pf(pf_ in){}
>
const char* operator()()
{
return ( m_p_class->*(m_pf))();
}
};
>
class my_error : public std::runtime_er ror {
public:
my_error(const std::string& msg) : std::runtime_er ror(msg) {}
>
};
>
>
int main()
{
my_error e("something" );
// need a class instance pointer and the member function pointer
// to init the functor
what_functor<my _errorfunc(&e,& my_error::what) ;
>
std::cout << func() <<'\n';
}
>

I know, it works, but why in my example it doesn't compile properly?


--
JRX --dev \dot jrx \at gmail \dot com
If the words "open source" get you more excited than
the words "free porn"...you might be a Game Developer.
Registered Linux user# 383163

[kwikius]

Re: Pointer to member function - type

jrx wrote:

I know, it works, but why in my example it doesn't compile properly?

It may be because your version has more template parameters and it
can't make asumptions therefore.. maybe someone will give a better
explanation. The following seems to give a similar message on gcc if
the DERIVED_OVERRID E define is commented out. So you could try
overriding the what function in your derived class. Note also the
throw() part of the signature which seems to be required here.

#include <stdexcept>
#include <iostream>

//comment out the define causes fail
#define DERIVED_OVERRID E
struct what_functor{
template <typename Class, typename R>
R operator()(Clas s * p_class, R (Class::* pf)()const)cons t
{
return ( p_class->*pf)();
}
};

class my_error : public std::runtime_er ror {
public:
my_error(const std::string& msg) : std::runtime_er ror(msg) {}
#ifdef DERIVED_OVERRID E
const char * what() const throw()
{
return std::runtime_er ror::what();
}
#endif
};

int main()
{
my_error e("something" );
what_functor func;

std::cout << func(&e,&my_err or::what) <<'\n';
}

regards
Andy Little

[jrx]

Re: Pointer to member function - type

kwikius wrote:

It may be because your version has more template parameters and it
can't make asumptions therefore.. maybe someone will give a better
explanation. The following seems to give a similar message on gcc if
the DERIVED_OVERRID E define is commented out. So you could try
overriding the what function in your derived class. Note also the
throw() part of the signature which seems to be required here.
>
#include <stdexcept>
#include <iostream>
>
//comment out the define causes fail
#define DERIVED_OVERRID E
struct what_functor{
template <typename Class, typename R>
R operator()(Clas s * p_class, R (Class::* pf)()const)cons t
{
return ( p_class->*pf)();
}
};
>
class my_error : public std::runtime_er ror {
public:
my_error(const std::string& msg) : std::runtime_er ror(msg) {}
#ifdef DERIVED_OVERRID Esame case as in my code. But I think it should compile even if method what() isn't overridden.
const char * what() const throw()
{
return std::runtime_er ror::what();
}
#endif
};
>
int main()
{
my_error e("something" );
what_functor func;
>
std::cout << func(&e,&my_err or::what) <<'\n';
}

That's the same case as in my code. But I think it should compile even
1if method what() isn't overridden. I have no idea, why me_error::what
behaves in such a strange way. Any suggestions for generic addMethod
method declaration?

--
JRX --dev \dot jrx \at gmail \dot com
If the words "open source" get you more excited than
the words "free porn"...you might be a Game Developer.
Registered Linux user# 383163