Re: Crazy Local Class

From:
"Chris M. Thomasson" <no@spam.invalid>
Newsgroups:
comp.lang.c++.moderated,comp.lang.c++
Date:
Sun, 26 Oct 2008 09:18:09 CST
Message-ID:
<Z_QMk.17102$UD6.10578@newsfe07.iad>
<cpluslearn@gmail.com> wrote in message
news:e730ba2e-6ac8-4063-945c-bcc06518e3fe@t54g2000hsg.googlegroups.com...

Hi,
    I have a local class inside a function template. I am able to wrap
any type in my local class. Is it legal C++? What type of class is
Local? Is it a class template or regular class?
    Thanks in advance.
--dhina
---------------------------------------------------------------------------------------------------------------------------------------------
class Foo
{
public:
virtual ~Foo() {}
virtual void print() = 0;
};

template< typename T >
Foo* wrap(const T& t)
{
class Local : public Foo
{
public:
Local(const T& t):val_(t) { }
void print()
{
cout << "In Local::print()" << val_ << endl;
}
private:
T val_;
};
return new Local(t);
}

int main(void)
{
int x = 50;
Foo* ptr = wrap(x);
ptr->print();

^^^^^^^^^^^^^^^^^^^

  delete ptr;

std::string s("wrap");
ptr = wrap(s);
ptr->print();

^^^^^^^^^^^^^^^^^^^

  delete ptr;

}


You have memory leaks. BTW, excuse me for being so dense, but what
advantages does this technique have over something like:
____________________________________________________________________
#include <iostream>
#include <string>

struct foo_impl {
  virtual ~foo_impl() = 0;
  virtual void print() const = 0;
};

foo_impl::~foo_impl() {
  std::cout << "In (" << this <<
    ")->foo_impl::~foo_impl()" << std::endl;
}

typedef foo_impl const& foo;

template<typename T>
class wrapper : public foo_impl {
  T m_obj;

  void print() const {
    std::cout << "In (" << this <<
      ")->wrapper<T>::print - " << m_obj << std::endl;
  }

public:
  wrapper(T const& obj)
    : m_obj(obj) {
  }

  ~wrapper() {
    std::cout << "In (" << this <<
      ")->wrapper<T>::~wrapper() - " << m_obj << std::endl;
  }
};

template<typename T>
static wrapper<T>
wrap(T const& obj) {
  return wrapper<T>(obj);
}

int main(void) {
  int x = 5;
  foo fx = wrap(x);
  fx.print();
  std::string s("wrap");
  foo fs = wrap(s);
  fs.print();
  return 0;
}
____________________________________________________________________

Thanks.

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