Re: [Boost-users] ptr_map trouble

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-04-14 03:25:04

Pau Garcia i Quiles skrev:
> Hello,
> 
> I'm having trouble when adding elements to a ptr_map. If I use a  
> ptr_vector instead, it works (code for ptr_map and ptr_vector follows).
> 
> * With ptr_map:
> ===============
> 
> #include <iostream>
> #include <string>
> #include <boost/function.hpp>
> #include <boost/any.hpp>
> #include <boost/ptr_container/ptr_map.hpp>
> 
> class Base { };
> 
> class X : public Base {
>    public:
>      std::string text() {  std::cout<< std::string("hello world!") <<  
> std::endl; return std::string("hello world!"); }
> };
> 
> class Y : public Base {
>    public:
>      int value() { return 5; }
> };
> 
> class Z : public Base {
>    public:
>      bool isEnabled() { return true; }
> };
> 
> // [...]
> // There will be hundreds of classes like X, Y and Z, only quite more complex
> 
> class ProxyBase {
>    public:
>      virtual boost::any field() = 0;
> };
> 
> template<class T>
> class Proxy : public ProxyBase {
>    typedef T result_type;
> 
>    public:
>      Proxy(T& inst, boost::function<boost::any ( T )> f) :  
> inst_(inst), f_(f) { ; };
> 
>      boost::any field() {
>          return boost::any( f_(inst_) );
>      }
> 
> 
>      T& inst_;
>    boost::function<boost::any ( T )> f_;
> 
> };
> 
> int main() {
> 
>    boost::ptr_map<std::string, ProxyBase> m;
> 
>    X x;
>    Proxy<X>* px = new Proxy<X>(x, boost::mem_fn(&X::text));
>    std::cout << "Calling a proxy object Proxy<X> outputs: " <<  
> boost::any_cast<std::string>( px->field() ) << std::endl;
> 
> //  m.insert(std::string("test"), px ); // FIXME Why does this 'insert' fail?

Please see the FAQ:

http://www.boost.org/doc/libs/1_35_0/libs/ptr_container/doc/faq.html#why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference

-Thorsten