I'm trying to call overloaded functions based on the derived type of a pointer in a vector.
I have a base class, Fruit, and create a vector of shared_ptr. I then push shared pointers of decendents into the vector. So far, so good (the issue comes in after I add code to print the elements).
Next, I want to iterate through the container, calling the appropriate Print functions for each fruit. I am getting compile errors:
# g++ -I/cygdrive/c/Program\ Files/boost/boost_1_52_0 -o main.exe main.cpp
main.cpp: In function `int main()':
main.cpp:80: error: no matching function for call to `Print_via_ptr(const boost::shared_ptr<Fruit>&)'
main.cpp:57: note: candidates are: void Print_via_ptr(boost::shared_ptr<Apple>)
main.cpp:58: note: void Print_via_ptr(boost::shared_ptr<Orange>)
main.cpp:59: note: void Print_via_ptr(boost::shared_ptr<Strawberry>)
main.cpp:95: error: no matching function for call to `Print(Fruit&)'
main.cpp:53: note: candidates are: void Print(const Apple&)
main.cpp:54: note: void Print(const Orange&)
main.cpp:55: note: void Print(const Strawberry&)
Here is the code:
#include <iostream>
#include <string>
#include <cstdlib>
#include <vector>
#include "boost/shared_ptr.hpp"
using std::cout;
using std::cin;
using std::endl;
struct Fruit
{
virtual const std::string& who_am_i(void) const = 0;
};
struct Apple
: public Fruit
{
const std::string& who_am_i(void) const
{
static const std::string name = "Apple";
return name;
}
};
struct Orange
: public Fruit
{
const std::string& who_am_i(void) const
{
static const std::string name = "Orange";
return name;
}
};
struct Strawberry
: public Fruit
{
const std::string& who_am_i(void) const
{
static const std::string name = "Strawberry";
return name;
}
};
void Pause(void);
void Print(const Apple& a);
void Print(const Orange& o);
void Print(const Strawberry& s);
void Print_via_ptr(boost::shared_ptr<Apple> p_a);
void Print_via_ptr(boost::shared_ptr<Orange> p_o);
void Print_via_ptr(boost::shared_ptr<Strawberry> p_s);
int main(void)
{
std::vector<boost::shared_ptr<Fruit> > basket;
boost::shared_ptr<Apple> p_apple(new Apple);
boost::shared_ptr<Orange> p_orange(new Orange);
boost::shared_ptr<Strawberry> p_strawberry(new Strawberry);
// Put fruit into basket.
basket.push_back(p_apple);
basket.push_back(p_orange);
basket.push_back(p_strawberry);
// Display the basket of fruit shared_ptr
cout << "Basket contents:\n";
for (std::vector<boost::shared_ptr<Fruit> >::const_iterator iter = basket.begin();
iter != basket.end();
++iter)
{
Print_via_ptr(*iter); // Line 80
}
// Create bowl of fruit pointers
std::vector<Fruit *> bowl;
bowl.push_back(new Apple);
bowl.push_back(new Orange);
bowl.push_back(new Strawberry);
// Print the fruit.
std::vector<Fruit *>::const_iterator bowl_iter;
for (bowl_iter = bowl.begin();
bowl_iter != bowl.end();
++bowl_iter)
{
Print(**bowl_iter); // Line 95
}
Pause();
return EXIT_SUCCESS;
}
void
Pause(void)
{
cout << "Paused. Press Enter to continue.\n";
cin.ignore(10000, '\n');
}
void
Print(const Apple& a)
{
cout << a.who_am_i() << endl;
}
void
Print(const Orange& o)
{
cout << o.who_am_i() << endl;
}
void
Print(const Strawberry& s)
{
cout << s.who_am_i() << endl;
}
void
Print_via_ptr(boost::shared_ptr<Apple> p_a)
{
cout << p_a->who_am_i() << endl;
}
void
Print_via_ptr(boost::shared_ptr<Orange> p_o)
{
cout << p_o->who_am_i() << endl;
}
void
Print_via_ptr(boost::shared_ptr<Strawberry> p_s)
{
cout << p_s->who_am_i() << endl;
}
I am on Windows Vista, using g++ under Cygwin:
# g++ --version
g++ (GCC) 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)
I want Print_via_ptr(boost::shared_ptr<Apple>)
to be called when I invoke:
Print_via_ptr(bowl[0]);
Polymorphic dispatch is done via a base pointer or reference type, it does not cast to derived pointer/reference type automatically.
std::vector<boost::shared_ptr<Fruit> > basket;
boost::shared_ptr<Apple> p_apple(new Apple);
basket.push_back(p_apple);
When you call push_back
, the objects are stored as boost::shared_ptr<Fruit>
not boost::shared_ptr<Apple>
They can only be called by
void Print_via_ptr(const boost::shared_ptr<Fruit> p_a);
^^^^^
Same issue with your raw pointer functions.
Or you could use template to generate calls for you, but this maybe is not your purpose.
template<typename T>
void Print_via_ptr(T p_o)
{
std::cout << p_o->who_am_i() << std::endl;
}