Modify Code through a GUI - c++

Question

Hey this is more of a question, i want to know if it is possible to modify code through GUI asking because i was asked to see if i could create a GUI where the user can change certain attributes. i.e an exmaple is below
start %= -(status)
> lexeme[elementV]
> -(lexeme[elementF])
> +(inboundGroup);
Above is part of my code which is Boost SPIRIT which parses Strings so for example would it be possible to change the + to a * or - etc
+ = One
- = optional
* = multiple
Do you think it would be possible to change that through a GUI i think it could be just not sure on how to do it?
Any help i will be very grateful
Thanks Shamari

Answer 1

Everything is possible in programming ;-)
For dynamic modification of a program during execution, there are several solutions :
Use a dynamic language like LUA
Use a plugin system with dynamic loading
Since you require C++ and Boost Spirit, I think the best solution is to generate a plugin on the fly and load it afterwards.
Your program will generate code, compile it into a shared library (.so) and then load and execute it. (Some people will find that dirty. It's insecure also. But it's simple and it works.)
Here is an exemple for linux : plugin.h :
#ifndef PLUGIN_H__
#define PLUGIN_H__
#ifdef __cplusplus
extern "C" {
#endif
int process();
typedef int (*plugin_process_fn_ptr)();
#ifdef __cplusplus
}
#endif
#endif // PLUGIN_H__
Note that we must use extern C or else, C++ name mangling will make it difficult to import symbols.
plugin.cpp :
#include "plugin.h"
#include <iostream>
using namespace std;
int process()
{
int return_value = 0;
#include "plugin_content.inc.cpp"
return return_value;
}
Note that I use a hack here, the code will be included from another file, "plugin_content.inc.cpp". The code from user will be put inside.
a script to build the plugin, "build_plugin.sh" :
#! /bin/sh
g++ -c -Wall -fPIC plugin.cpp -o plugin.o
gcc -shared -o libplugin.so plugin.o
Now the calling program, main.cpp :
#include <iostream>
#include <fstream> // to open files
#include <dlfcn.h> // C lib to load dynamic libs
#include "plugin.h"
using namespace std;
// load the plugin and call the process() function fom it
static int process_via_plugin()
{
int return_value = -1;
void *lib_handle(NULL);
char *error(NULL);
char *plugin_lib = "./libplugin.so";
lib_handle = dlopen(plugin_lib, RTLD_LAZY);
if (!lib_handle)
{
cerr << "Error loading lib " << plugin_lib << " : " << dlerror() << endl;
exit(1);
}
char *plugin_fn = "process";
plugin_process_fn_ptr fn = (plugin_process_fn_ptr)dlsym(lib_handle, plugin_fn);
error = dlerror();
if (error)
{
cerr << "Error finding lib " << plugin_fn << " : " << error << endl;
exit(1);
}
// call the function loaded from lib
return_value = (*fn)();
dlclose(lib_handle);
lib_handle = NULL; // useless but for good habits ^^
return return_value;
}
// build or rebuild the plugin,
// we must call it when we change the plugin code code
static int build_plugin(string code)
{
{
char *plugin_code_file = "plugin_content.inc.cpp";
ofstream plugin_code(plugin_code_file, ios::out);
plugin_code << code << endl;
}
system("build_plugin.sh");
return 0;
}
// our program
int main(int argc, char *argv[])
{
cout << "Hello World !" << endl;
string code = ""
"cout << \"Hello from plugin !\" << endl;"
"";
// build a first version of the plugin and call it
build_plugin(code);
process_via_plugin();
// now we modify the code (use a GUI here)
code = ""
"cout << \"Hello from plugin, updated !\" << endl;"
"";
// rebuild the plugin and call it again
build_plugin(code);
process_via_plugin();
// do it again as much as you want.
return 0;
}
Now, build your program :
g++ -Wall -rdynamic -ldl main.cpp
and execute it :
a.out
and you get :
Hello World !
Hello from plugin !
Hello from plugin, updated !
The code I give you is very basic. For example, we should check if the compilation of the plugin is successful and report errors to the user. Now it's up to you to add more stuff.