Thread: Executing C++ Code at Runtime

Hi everybody,
I'm looking for a way to let the user enter their own bit of code into the program, and then have the program evaluate that line. As a web designer I know that JavaScript has such a function, which follows this syntax:

Code:

var string = "xyz = 1 + 3";
eval(string);

That code will assign the value 4 to the variable xyz.

Is there any equivalent way to do this in C++? Ultimately, I want to create a program that lets the user enter a function, and then the program will find the derivative of that function (but, of course, there are many other uses as well).

Any help is appreciated!

>Is there any equivalent way to do this in C++?
Sure, but it involves writing your own C++ interpreter. Somehow I get the feeling you don't want to do that.

>Just because it's not standard doesn't mean it isn't existant.
*Flips through the C++ standard*

I can't find that clause anywhere.

>It's on page 783...
When referring to the standard it's customary to quote the relevant part that makes your point. So let's see it.

Oh... you were being serious... I thought you were just kidding...

No, that's not written in the standard anywhere, but take a look at SDL. SDL is not standard, but it's there. That's all I was saying. Just because there isn't a standard function that executes the code contained in a string, doesn't mean someone, somewhere hasn't written one.

edit: And for anyone that doesn't get the joke about Page 783 - There are only 782 pages in the standard. Also the reason for a quick PM to Prelude to find that out.

>Oh... you were being serious... I thought you were just kidding...
I'm never serious, but I can't resist shooting down a good joke.

http://www.softintegration.com/

C++ would be my last choice as a script language.

If your using Windows, you can make use of the MS Script Control to run your script in either VBScript or JScript. Check out DispHelper, especially the script control sample.

Here is a sample of executing javascript:

Code:

int nResult;
RunScript(L"%d", &nResult, TEXT("Math.round(Math.pow(5, 2) * Math.PI)"), TEXT("JScript"));
cout << "Result = " << nResult << endl;

You could execute your script:

Code:

int nResult;
RunScript(L"%d", &nResult, TEXT("1 + 3"), TEXT("JScript"));
cout << "Result = " << nResult << endl;

that would of course require the user to have a compiler

You might be more interested in looking at some interpreted language such as Scheme (for which interpreters do exist), or something of your own design. It would be far too much hassle to try to make C++ into a scripting language, in my opinion (especially for a math program like that).

Cheers

eval(string)

isn't there a bunch of security risks associated with this also? having the end user excute arbitarty code like that. Is there some sandbox in place?

I'm not sure that Javascript can get into anything terribly vital, but I do know that such a thing would be rather unwise in something like Perl. Similar to one of the arguments against using the system( ) command in C++.

jdm, check this thread: http://cboard.cprogramming.com/showthread.php?t=57927
It is pertinent to the problem you are trying to solve.