Is there any way to compile additional code at runtime in C or C++?

Question

Here is what I want to do:

1. Run a program and initialize some data structures.
2. Then compile additional code that can access/modify the existing data st

Answer 1

There is one simple solution:

1. create own library having special functions
2. load created library
3. execute functions from that library, pass structures as function variables

To use your structures you have to include same header files like in host application.

structs.h:

struct S {
    int a,b;
};

main.cpp:

#include 
#include 
#include 
#include 

#include "structs.h"

using namespace std;

int main ( int argc, char **argv ) {

    // create own program
    ofstream f ( "tmp.cpp" );
    f << "#include\n#include \"structs.h\"\n extern \"C\" void F(S &s) { s.a += s.a; s.b *= s.b; }\n";
    f.close();

    // create library
    system ( "/usr/bin/gcc -shared tmp.cpp -o libtmp.so" );

    // load library        
    void * fLib = dlopen ( "./libtmp.so", RTLD_LAZY );
    if ( !fLib ) {
        cerr << "Cannot open library: " << dlerror() << '\n';
    }

    if ( fLib ) {
        int ( *fn ) ( S & ) = dlsym ( fLib, "F" );

        if ( fn ) {
            for(int i=0;i<11;i++) {
                S s;
                s.a = i;
                s.b = i;

                // use function
                fn(s);
                cout << s.a << " " << s.b << endl;
            }
        }
        dlclose ( fLib );
    }

    return 0;
}

output:

0 0
2 1
4 4
6 9
8 16
10 25
12 36
14 49
16 64
18 81
20 100

You can also create mutable program that will be changing itself (source code), recompiling yourself and then replace it's actual execution with execv and save resources with shared memory.