Boost Mem_fn and accessing member function of derived class - boost

Question

I made a simple example to test boost bind's interaction with derived classes.
I created two subclasses with different getarea functions. I expected
g1 = boost::bind(boost::mem_fn(&Shape::getarea), Rec)
to print the area of Rectangle(10,20) but instead it printed '1'. I get the same when I instead write Rectangle::getarea. It prints the same even when I input other functions eg. member of Rectangle
double sum(double h,double w){return h+w; }
and use
g1 = boost::bind(boost::mem_fn(&Rectangle::sum), Rec,2,3)
Question 1: Why does it return '1'?Is that a default response for error?
My second problem is to do the same of printing g2 but now Rec is replaced by **iter, i.e. an object of some derived class type from a list of objects. Since getarea is a virtual fcn, once I get the above working it should be fine to just write:
g2= boost::bind(boost::mem_fn(& Shape::getarea , &(**iter));
Question 2: However, I was wondering if there is a way to return the classtype of **iter eg. classof(**iter) and then put it in g2 i.e.
g2= boost::bind(boost::mem_fn(& classof(**iter)::getarea , &(**iter));
When I ran g2 by writing Shape::getarea, I got '1' again for all iter.
#include <memory>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <boost/bind.hpp>
using namespace std;
class Shape {
public:
Shape(double h, double w) :height(h), width(w) {};
virtual double getarea() = 0;
double height;
double width; };
class Rectangle: public Shape {
public:
Rectangle(double h, double w): Shape(h,w) {};
double getarea() override { return height*width; } };
class Triangle : public Shape {
public:
Triangle(double h, double w) :Shape(h,w) {};
double getarea() { return height*width*0.5; }};
int main() {
//create objects
Rectangle Rec(10, 20);
Triangle Tri(2, 3);
//create boost bind function
boost::function<double(double, double)> g1;
g1 = boost::bind(boost::mem_fn(&Shape::getarea), Rec);
//print area and g
cout << Rec.getarea()<<" should be equal to " << g1<< '\n';
//create list
vector<shared_ptr<Shape>> Plist;
Plist.push_back(make_shared<Rectangle>(Rec));
Plist.push_back(make_shared<Triangle>(Tri));
//print each element from the vector list
for (auto iter = Plist.begin(); iter != Plist.end(); iter ++ ) {
boost::function<double(double, double)> g2;
g2= boost::bind(boost::mem_fn(& .... , &(**iter));
//where in dots we need Classtype_of_**iter::getarea
cout << (**iter).getarea()<<"should be equal to " << g2<< '\n';
}
}

Answer 1

You... forget to invoke the functions...
for (auto iter = Plist.begin(); iter != Plist.end(); iter++) {
boost::function<double()> g2;
g2 = boost::bind(&Shape::getarea, iter->get());
cout << (*iter)->getarea() << " should be equal to " << g2() << '\n';
}
What you saw what the implicit conversion to bool (http://www.boost.org/doc/libs/1_60_0/doc/html/boost/function.html#idm45507164686720-bb)
Note also I fixed the signature of g1 and g2: Live On Coliru.
Some further improvements (remove the need for the g2 in the loop?):
auto getarea = boost::mem_fn(&Shape::getarea);
for (auto iter = Plist.begin(); iter != Plist.end(); iter++) {
cout << (*iter)->getarea() << " should be equal to " << getarea(**iter) << '\n';
}
Or, indeed in c++11:
for (auto& s : Plist)
cout << s->getarea() << " should be equal to " << getarea(*s) << '\n';
By this time, you'd wonder why you have this accessor when you can just use the member.