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Re: How does a lambda actually get assigned to a std::function<>?

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On 14.01.2012 21:42, K. Frank wrote:
>
> I'm looking for some insight into how an assignment such as:
>
>     std::function<int, (int, int)>  f = [z](int x, int y){ return x + y
> + z; };
>
> actually works under the hood.

Well, disregarding "actually", here's one possibility for your specific 
example:


<code>
#include <iostream>
#include <vector>
#include <memory>       // std::shared_ptr

class WrapperBase
{
public:
     virtual ~WrapperBase() {}
};

template< class Functor >
class Wrapped
     : public WrapperBase
{
public:
     Functor f;

     Wrapped( Functor const& _f ): f( _f ) {}
};

class MyFunc
{
private:
     typedef int (*InvokerFunc)( void*, int, int );

     std::shared_ptr< WrapperBase >      pFunc_;
     InvokerFunc                         invoker_;

     template< class Functor >
     static int invoke( void* pF, int x, int y )
     {
         return (reinterpret_cast< Wrapped< Functor>* >( pF )->f)( x, y );
     }

public:
     template< class Functor >
     MyFunc( Functor const& f )
         : pFunc_( new Wrapped< Functor >( f ) )
         , invoker_( &invoke< Functor > )
     {}

     int operator()( int x, int y ) const
     {
         return invoker_( pFunc_.get(), x, y );
     }
};

int main()
{
     using namespace std;

     MyFunc  f = []( int x, int y ) -> int { return x*y; };
     cout << f( 6, 7 ) << endl;
}
</code>


The technique used for the invoker function is called "type erasure", 
and I believe it was developed (by the Boost guys) for just this 
purpose, namely for the implementation of boost::function.


Cheers & hth.,

- Alf

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Thread

How does a lambda actually get assigned to a std::function<>? "K. Frank" <kfrank29.c@gmail.com> - 2012-01-14 12:42 -0800
  Re: How does a lambda actually get assigned to a std::function<>? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-01-14 22:24 +0100
    Re: How does a lambda actually get assigned to a std::function<>? "K. Frank" <kfrank29.c@gmail.com> - 2012-01-14 19:14 -0800
      Re: How does a lambda actually get assigned to a std::function<>? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-01-15 14:27 +0100
    Re: How does a lambda actually get assigned to a std::function<>? Peter Remmers <p.remmers@expires-2011-04-30.arcornews.de> - 2012-01-15 12:16 +0100
      Re: How does a lambda actually get assigned to a std::function<>? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-01-15 14:24 +0100
        Re: How does a lambda actually get assigned to a std::function<>? Peter Remmers <p.remmers@expires-2011-04-30.arcornews.de> - 2012-01-16 01:52 +0100
          Re: How does a lambda actually get assigned to a std::function<>? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-01-16 05:32 +0100
  Re: How does a lambda actually get assigned to a std::function<>? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-01-14 22:27 +0100
    Re: How does a lambda actually get assigned to a std::function<>? "K. Frank" <kfrank29.c@gmail.com> - 2012-01-14 13:54 -0800

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