Question

This is what I’m trying to do (this is a simplification of a real project):

int param;
int result;
void isolated(int p) {
  param = p;
  try {
    // make calculations with "param" and place the
    // result into "result"
    process();
  } catch (...) {
    throw "problems..";
  }
}

I can’t change the way process() works, since this function is not created in the project and is a third-party function. It works with global variables param and result and we can’t change this.

The problem appears when isolated() is called back from process() with another parameter. I want to catch this situation, but don’t know how to do it, since finally is absent in C++. I feel that I should use RAII technique, but can’t figure out how to do it in this case properly.

This is how I can make it with code duplication:

int param;
int result;
void isolated(int p) {
  static bool running;
  if (running) {
    throw "you can't call isolated() from itself!";
  }
  running = true;
  param = p;
  try {
    // make calculations with "param" and place the
    // result into "result"
    process();
    running = false;
  } catch (...) {
    running = false; // duplication!
    throw "problems..";
  }
}

Answer 1

I still don’t quite sure how finally is related here, but you could try Boost.ScopeExit if you want to avoid creating a scope guard structure yourself.

Example:

#include <boost/scope_exit.hpp>
#include <cstdio>

int isolated(int p) {
    static bool running = false;
    if (running) {
        printf("Throwing %d\n", p);
        throw p;
    }
    printf("Starting %d\n", p);
    running = true;
    BOOST_SCOPE_EXIT( (p)(&running) ) {   // <--
        printf("Stopping %d\n", p);       // <--
        running = false;                  // <--
    } BOOST_SCOPE_EXIT_END                // <--

    // ...
    if (p)
        isolated(p*10);
    // ...
    printf("Returing %d\n", p);

    return 4;
}

int main() {
    printf(">> first\n");
    isolated(0);
    printf(">> second\n");
    try {
        isolated(1);
        printf(">> third (should not be printed.)\n");  
    } catch(int p) {
        printf("Caught %d\n", p);
    }
    isolated(0);
    printf(">> fourth\n");

    return 0;
}

Result:

>> first
Starting 0
Returing 0
Stopping 0
>> second
Starting 1
Throwing 10
Stopping 1
Caught 10
Starting 0
Returing 0
Stopping 0
>> fourth