Determine if a class has a nested class defined

[Martin]

Hi.

Is it possible to determine if a certain class has a nested class/
struct defined at compile time?

E.g., we have:

class A
{
public:
class B {};
};

Write a small program, which prints out if class A has a nested class
defined, or not, of course determined at compile time.

Thanks in advance,
Martin

 

[Kai-Uwe Bux]

Hi.

Is it possible to determine if a certain class has a nested class/
struct defined at compile time?
Yes.

E.g., we have:

class A
{
public:
class B {};
};

Write a small program, which prints out if class A has a nested class
defined, or not, of course determined at compile time.

Sounds like homework.

Also, do you want to check for any member type / typedef? or do you insist
on B being a class. E.g., what shall the program print for

class A {
public:
typedef int B;
};


Best

Kai-Uwe Bux

 

[Martin]

Sounds like homework.

Also, do you want to check for any member type / typedef? or do you insist
on B being a class. E.g., what shall the program print for

class A {
public:
  typedef int B;

};

Best

Kai-Uwe Bux

Yes, this is a homework I don't insist on 'B' being just a class
type - please just bring an example, I'll catch the idea of how to
extend it for other cases

 

[James Kanze]

Yes, this is a homework I don't insist on 'B' being just a
class type - please just bring an example, I'll catch the idea
of how to extend it for other cases

Which is doubtlessly why Kai-Uwe didn't go into more detail. We
do occasionally help with homework, but you have to make the
first step yourself. Try looking up SFINAE. (The ultimate
source for learning templates is, of course the Vandevoorde and
Jussutis. If your course is dealing with advanced topics like
this, the professor should have already suggested it.)

 

[yurec]

Hi

reading http://www.semantics.org/once_weakly/w02_SFINAE.pdf

i wrote this, but it seems too ugly as for me. is there better way?

#include <iostream>

class A
{
public:
class B {};
};


#define ClassHasInnerClass(Class, InnerClass) \
class Class##Has##InnerClass \
{ \
typedef char True; \
typedef struct {char test[2];} False; \
\
template <typename Class> \
static False _has(...); \
\
template <class Class> \
static True _has( typename Class::InnerClass const * ); \
\
public: \
enum {Result = (sizeof(_has<Class>(0)) == sizeof(True))}; \
};

ClassHasInnerClass(A,B);


int main()
{
if (AHasB::Result)
std::cout << "has" << std::endl;
else
std::cout << "has not" << std::endl;

getchar();
return 0;
}

 

[James Kanze]

readinghttp://www.semantics.org/once_weakly/w02_SFINAE.pdf

i wrote this, but it seems too ugly as for me. is there better way?

Template metaprogramming is often a bit ugly, but not
necessarily moreso than macros.

#include <iostream>

class A
{
public:
class B {};
};

#define ClassHasInnerClass(Class, InnerClass) \
class Class##Has##InnerClass \
{ \
typedef char True; \
typedef struct {char test[2];} False; \
\
template <typename Class> \
static False _has(...); \
\
template <class Class> \
static True _has( typename Class::InnerClass const * ); \
\
public: \
enum {Result = (sizeof(_has<Class>(0)) == sizeof(True))}; \
};

What's wrong with:

ClassHasInnerClass(A,B);

In which case, you don't need this.

int main()
{
if (AHasB::Result)

And this becomes:

if ( HasInner< A, B >::result )

(I might use macros to clean up this statement, e.g. something
like

#define hasInner(a,b) HasInner< a, b >::result

but that's about all.)

 

[yurec]

readinghttp://www.semantics.org/once_weakly/w02_SFINAE.pdf
i wrote this, but it seems too ugly as for me. is there better way?

Template metaprogramming is often a bit ugly, but not
necessarily moreso than macros.

#include <iostream>
class A
  {
  public:
    class B {};
  };
#define ClassHasInnerClass(Class, InnerClass)                       \
class Class##Has##InnerClass                                        \
  {                                                                 \
  typedef char True;                                                \
  typedef struct {char test[2];} False;                             \
                                                                    \
  template <typename Class>                                         \
  static False _has(...);                                           \
                                                                    \
  template <class Class>                                            \
  static True _has( typename Class::InnerClass const * );           \
                                                                    \
  public:                                                           \
    enum {Result = (sizeof(_has<Class>(0)) == sizeof(True))};       \
  };

What's wrong with:

ClassHasInnerClass(A,B);

In which case, you don't need this.

int main()
  {
  if (AHasB::Result)

And this becomes:

    if ( HasInner< A, B >::result )

(I might use macros to clean up this statement, e.g. something
like

#define hasInner(a,b) HasInner< a, b >::result

but that's about all.)

    std::cout << "has" << std::endl;
  else
    std::cout << "has not" << std::endl;

  getchar();
  return 0;
  }

--
James Kanze (GABI Software)             email:[email protected]
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"HasInner< A, B >::result" gives compile error 'B' : undeclared
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