Re: [Boost-Users] boost::bind/function and C function pointers

From: Thomas Pollinger (tpolling_at_[hidden])
Date: 2003-02-12 12:53:13

>
>
>>Probably more as a site note (curiosity if you will on a language level):
>>
>>#include <boost/function.hpp>
>>
>>int someFoo(int val) {return val + 5;}
>>
>>void someMain() {
>>    boost::function<int (int)> bptr = someFoo;
>>    int (*cptr)(int) = someFoo;
>>    bptr(5);
>>    cptr(5);
>>    if (bptr == cptr) printf("equal\n");
>>}
>>
>>The above code snipet does not compile as the types of bptr and cptr are
>>not the same and thus their values cannot be compared although they are
>>identical.
>>    
>>
>
>Even if bptr and cptr were both boost::function instances of the same type 
>they would not compare. 
>
OK.

>
>  
>
>>I have not followed the C++ Spec evolution closely and am only familar
>>with what appears to be the 98 spec. Are there any plans to uniformize
>>this on the language level? This would allow interfacing C style code
>>with the nice boost function pointers in a much cleaner way.
>>    
>>
>
>The limitation is not a language limitation but a library limitation. One 
>cannot compare two boost::function objects because one cannot necessarily 
>compare two arbitrary function objects, and even simpler versions (e.g., 
>requiring that the boost::function targets have the same type) run into 
>trouble.
>
True for comparison. However a C function pointer and a boost::function 
pointer with the same signature virtually have the same type. One would 
expect that a boost::function pointer should be castable to a C style 
pointer, as you can have std::string return a const char * pointer to 
its C style string.

E.g. from the code snipet example:
int (*cptr)(int) = bptr.fct_ptr(); (or some similar member).

In the case of a bound function pointer, fct_ptr should arrange to 
return a pointer pointing to the function's closure, comprising thus the 
member function pointer plus eventually bound argument(s).

The problem certainly is that boost::function is itself a class and 
you're handling instances of that class. Any access to its instance data 
has to happen through its members which seems to rule out the 
possibility of returning a static or C function compatible type with 
fct_ptr(). That is why I was not sure whether there is something missing 
in C++ where you can bind a member function to an instance of its 
declaring class and as a result getting a C-style  compatible function 
pointer.

-Thomas