Re: [Boost-users] boost::mpl::switch?

From: Robert Dailey (rcdailey_at_[hidden])
Date: 2008-05-02 09:44:23

On Fri, May 2, 2008 at 12:45 AM, Agustín K-ballo Bergé <
kaballo86_at_[hidden]> wrote:

> Your first thoughts on a solution using a mpl::map are valid. All you
> need to do is create a free function pointer template to wrap the calls.
> The following code shows how:
>
> #include <iostream>
>
> #include <boost/mpl/map.hpp>
> #include <boost/mpl/at.hpp>
>
> void SetParamFloat( char* name, float value )
> {
>        std::cout << "SetParamFloat called: " << name << ", " << value <<
> std::endl;
> }
>
> void SetParamInt( char* name, int value )
> {
>        std::cout << "SetParamInt called: " << name << ", " << value <<
> std::endl;
> }
>
> void SetParamString( char* name, char* value )
> {
>        std::cout << "SetParamString called: " << name << ", " << value <<
> std::endl;
> }
>
> template< typename T, void ( *ptr_to_function )( char*, T ) > class
> free_function_ptr
> {
> public:
>        static void call( char* name, T value )
>        {
>                ptr_to_function( name, value );
>        }
> };
>
> typedef boost::mpl::map<
>        boost::mpl::pair< float, free_function_ptr< float, SetParamFloat >
> >,
>        boost::mpl::pair< int, free_function_ptr< int, SetParamInt > >,
>        boost::mpl::pair< char*, free_function_ptr< char*, SetParamString >
> >
>  >::type functions_map;
>
> template < typename T > void set_param( char* name, T value )
> {
>        typedef boost::mpl::at< functions_map, T >::type function_;
>
>        function_::call( name, value );
> }
>
> int main()
> {
>        set_param( "float", 1.3f );
>
>        set_param( "int", 1 );
>
>        char *text = "property value";
>        set_param( "char*", text );
>
>        return 0;
> }
>
> Running the code will output:
>     SetParamFloat called: float, 1.3
>     SetParamInt called: int, 1
>     SetParamString called: char*, property value
>
> I have compiled the code in MS VC++ 9 and the machine code generated is
> the same as calling the actual functions directly (actually, the
> provided test functions are inlined).
>
> The shortcome of this solutions is that special care should be taken
> when specifying the value. For instance, set_param( "float", 1.3 );
> won't work since the value provided is a double, and there is no entry
> for double at the function map. A quick (and tedious) solution would be
> to add more entries to the map. Anyone can think of a better solution?

You'd think the compiler would be smart enough to implicitly call the float
version, but then again who's to say it doesn't pick the integer version
either?