Multithreading in C++

To use the multithreaded garbage collector, include gcthrd.h in your program. Then, use GCPtr in the same way as described in Chapter 2. When you compile the program, you must remember to link in the multithreaded libraries, as explained earlier in this chapter in the section describing _beginthreadex( ) and endthreadex( ).

To see the effects of the multithreaded garbage collector, try this version of the load test program originally shown in Chapter 2:

// Demonstrate the multithreaded garbage collector. #include <iostream>
#include <new>
#include "gcthrd.h"
using namespace std;
// A simple class for load testing GCPtr.
class LoadTest {
  int a, b;
public:
  double n[100000]; // just to take-up memory
  double val;
  LoadTest() { a = b = 0; }
  LoadTest(int x, int y) {
    a = x;
    b = y;
    val = 0.0;
  }

  friend ostream &operator<(ostream &strm, LoadTest &obj);
};
// Create an insertor for LoadTest.
ostream &operator<(ostream &strm, LoadTest &obj) {
  strm << "(" << obj.a << " " << obj.b << ")";
  return strm;
}
int main() {
  GCPtr<LoadTest> mp;
  int i;
  for(i = 1; i < 2000; i++) {
    try {
      mp = new LoadTest(i, i);
      if(!(i%100))
        cout << "gclist contains " << mp.gclistSize()
             << " entries.\n";
    } catch(bad_alloc xa) {
      // For most users, this exception won't
      // ever occur.
      cout << "Last object: " << *mp << endl;
      cout << "Length of gclist: "
           << mp.gclistSize() << endl;
    }
  }
  return 0;
}

Here is a sample run. (Of course, your output may vary.) This output was produced with the display option turned on by defining DISPLAY within gcthrd.h.

Garbage collection started.
gclist contains 42 entries.
gclist contains 35 entries.
gclist contains 29 entries.
gclist contains 22 entries.
gclist contains 18 entries.
gclist contains 11 entries.
gclist contains 4 entries.
gclist contains 51 entries.
gclist contains 47 entries.
gclist contains 40 entries.
gclist contains 33 entries.
gclist contains 26 entries.
gclist contains 19 entries.
gclist contains 15 entries.
gclist contains 10 entries.
gclist contains 3 entries.
gclist contains 53 entries.
gclist contains 46 entries.
gclist contains 42 entries.
Before collecting for shutdown() for class LoadTest
After collecting for shutdown() for class LoadTest

As you can see, because collect( ) is running in the background, gclist never gets very large, even though thousands of objects are being allocated and abandoned.

Some Things to Try

Creating successful multithreaded programs can be quite challenging. One reason for this is the fact that multithreading requires that you think of programs in parallel rather than linear terms. Furthermore, at runtime, threads interact in ways that are often difficult to anticipate. Thus, you might be surprised (or even bewildered) by the actions of a multithreaded program. The best way to get good at multithreading is to play with it. Toward this end, here are some ideas that you might want to try.

Try adding another list box to the thread control panel that lets the user adjust the priority class of the thread in addition to its priority value. Try adding various synchronization objects to the control panel that can be turned on or off under user control. This will let you experiment with different synchronization options.

For the multithreaded garbage collector, try collecting garbage less often, such as when gclist reaches a certain size or after free memory drops to a predetermined point. Alternatively, you could use a waitable timer to activate garbage collection on a regular basis. Finally, you might want to experiment with the garbage collector’s priority class and settings to find which level is optimal for your use.