On Friday, December 21, 2012 09:32 AM, Lars Viklund wrote:
Hello,
I have got a a C library function call, which defines a function: typedef int (*CFunction) (State* K);
So I would like to use boost::bind / boost::function to create the binding to a class method:
boost::function<CFunction>( boost::bind(&MyClass::myMethod, this) )
so that I can use the class method with the CFunction signature. I would like to call a class method with the object context like a C function.
Can anybody help me to create a correct binding? Within the standard language, no, there is no way to get a free function
On Fri, Dec 21, 2012 at 12:30:20AM +0100, Philipp Kraus wrote: pointer to an arbitrary callable.
The result of a bind expression is not a free function pointer nor convertible to one. It's an object of an unspecified class type with a suitable operator ().
You have several horrible choices for getting a free function pointer from something like this.
Stateless C++11 lambdas can be coerced into a free function pointer, but as they're stateless, they will not fly here.
The common within-the-language solution is to make a free "trampoline" function that forwards calls to your object, which it retrieves from some secret hiding place. In some APIs, you can pass in a "context" or "state" object at the same place you provide the free function pointer to the callback:
R trampoline(State* s, A0 a0, A1 a1) { return get_my_callable(s)(a0, a1); }
register_callback_with_api(&trampoline, &my_callable);
Another choice that is not within the standard is to generate machine code for a trampoline-like function that directly forwards your call, either through some code generation library like Xbyak or by taking a suitable function, copying the generated code and patch the parts where the call happens.
The above is often called "thunking".
If the State structure has a void *user_data field, then you can use the
following:
template
In summary, the ways in order of unhorribleness are:
1) squirrel the actual callable away in some context/state provided to the trampoline;
2) squirrel the actual callable away in some global or static variable, which the trampoline uses;
3) dive into the abyss of code/thunk generation.
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