Dynamic source code in C++

Question

How to process dynamic source code in C++? Is it possible to use something like eval("foo")?

I have some functions that need to be called depending on user's choice:

     void function1 ();
     void function2 ();
     ...
     void functionN ();

     int main (int argv, char * argv [])
     {
         char * myString = new char [100];
         ...
         myString = "1" //user input            
         cout << eval("function"+myString)();
     }

How is it usually done?

UPD: Based on slacy's and clinisbut's answers I think I need to make a function registry. I suppose it should be made as an array of pointers to functions. Here's the question, how do I declare an array of pointers to functions?

Answer 1

C++ is a compiled language, and thus, there is no equivalent to "eval()".

For the specific example you mention, you could make a function registry that maps inputs (strings) to outputs (function pointers) and then call the resultant function.

There are several C++ interpreter libraries that are available, although performance will be very poor, they may accomplish what you want. Google search for "C++ interpreter". I've seen results for "Ch", "CINT" and "clipp"

Answer 2

You can't do that in C++. It is a mandatorily-compiled language.

A lot of information that would be necessary to implement eval() is lost at run-time anyway.

Answer 3

You could embed a C/C++ interpreter into your program if you really need this. However, you could also embed a more script-like language instead.

Answer 4

To answer your question of how this is usually done, the answer is: it isn't.

Having to use something like eval() is usually a design problem. If you really do need it, embed a scripting language such as Lua or Python.

Answer 5

I would build an array, or an STL list, of function pointers and the strings that you find that trigger the call to them. If you really need to call an arbitrary function you should investigate RTTI http://en.wikipedia.org/wiki/Run-time_type_information

Answer 6

If you put function1 .. functionN in a DLL, you could simply pull them out and invoke them by name using dlopen/dlsym which would probably get you 90% of the way you want to go.

Answer 7

The C++ language itself is not able to do that directly. But you may use plateform specific functions to call any function in a given DLL, according to user input (or whatever). For example, on Windows, look for GetProcAddress() and LoadLibrary()

Answer 8

The real answer to your question is this:

extern "C" {
void function1 ();
void function2 ();
void function3 ();
}


 int main (int argv, char * argv [])
 {
     char * myString = new char [100];
     ...
     myString = "function1"; //user input     

     void (*f)() = dlsym(RTLD_NEXT, myString);
     f();
 }

You can obtain the function defined in your binary (by its name, if it was declared with extern "C"), and call it.

On windows, it becomes uglier, but still possible - read on GetProcAddress

Answer 9

This is a good case for embedding an interpreted language like LUA. If you create LUA bindings to your C++ API, you'll be able to dynamically execute your code via LUA code from your application.

Answer 10

How about function pointers?

Answer 11

What you want is reflection. It's not supported by default in C++, but many people have written comprehensive reflection systems for all sorts of purposes. One that comes to mind off the top of my head is binding C++ functions to a custom scripting language.

Others have mentioned using a switch statement (or more accurately and if/else tree). This is about as simple as a reflection system is likely to come... and it is probably the best solution for your project. Reflection in C++ is a fairly advanced technique and often requires a lot of additional code... sometimes even custom preprocessors.

Also note that for security purposes, eval() is almost never something you want to be using in any language.

Answer 12

how do I declare an array of pointers to functions?

The C programmer's response to this question:

use cdecl.

cdecl> declare functable as pointer to function (int,pointer to int) returning double

double (*functable)(int , int *)

cdecl> declare functable as array 17 of pointer to function (int) returning double

double (*functable[17])(int )

Note that functions automatically convert into pointers to functions, in the same sense that arrays decay into pointers. You don't need an & to take the address of a function to put it in a pointer.

If you're in C++, it might help to use a std::map<YourFunctionPointerType> instead of an array, since you want to map strings anyway...

The OO approach:

Use polymorphism instead. It doesn't immediately solve your problems, but there's a good chance that if you're doing this it makes sense to have a class for each function.

Note that a vtable is, fundamentally, a table of pointers to functions.