Re: problem using for_each with member function of abstract class

From:

Bart van Ingen Schenau <bart@ingen.ddns.info>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 13 Feb 2009 12:23:42 CST

Message-ID:

<1298476.peDKFck9HY@ingen.ddns.info>

piwi wrote:

Hello,

As shown in the sample below, I have a vector of a POD in a class.
This class is abstract. I want to implement a print method for the
vector, but managed at the class' level, and using the for_each
algorithm.

struct Bar { /* ... */ };

class Foo
{
public:
  Foo() {}
  virtual ~Foo() {} = 0

public:
  void print(FILE* output) const;
private:
  void printBar(Bar const& bar) const;

private:
  std::vector<Bar> _v;
};

void
Foo::print(FILE* output)
{
  std::for_each(_v.begin(), _v.end(), std::bind1st(std::mem_fun_ref
(&Foo::printBar), *this));
}

Sooo.... Here is my problem: I cannot provide *this in the line above,
because it causes the compiler to fail (reference to reference is
illegal). I cannot instantiate Foo as-is because it is abstract. And I
cannot put printBar in struct Bar because I have specific processing
that is handled by Foo's derived classes.

Is there any workaround that would allow me to call Foo::printBar in
the for_each algorithm?

Yes. You can write Foo::print like this:

  void
  Foo::print(FILE* output)
  {
    std::for_each(_v.begin(), _v.end(), std::bind1st(std::mem_fun
  (&Foo::printBar), this));
  }

And I had to change printBar to take the Bar argument by value, instead
of const reference, but that could be the result of a bug in my
implementation (GCC+libstdc++)

Thanks !

Bart v Ingen Schenau
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