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Thinking in C++ Vol 2 - Practical Programming
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Thread local storage

A second way to eliminate the problem of tasks colliding over shared resources is to eliminate the sharing of variables, which can be done by creating different storage for the same variable, for each different thread that uses an object. Thus, if you have five threads using an object with a variable x, thread local storage automatically generates five different pieces of storage for x. Fortunately, the creation and management of thread local storage is taken care of automatically by ZThread s ThreadLocal template, as seen here:

//: C11:ThreadLocalVariables.cpp {RunByHand}
// Automatically giving each thread its own storage.
//{L} ZThread
#include <iostream>
#include "zthread/Thread.h"
#include "zthread/Mutex.h"
#include "zthread/Guard.h"
#include "zthread/ThreadedExecutor.h"
#include "zthread/Cancelable.h"
#include "zthread/ThreadLocal.h"
#include "zthread/CountedPtr.h"
using namespace ZThread;
using namespace std;
class ThreadLocalVariables : public Cancelable {
ThreadLocal<int> value;
bool canceled;
Mutex lock;
ThreadLocalVariables() : canceled(false) {
void increment() { value.set(value.get() + 1); }
int get() { return value.get(); }
void cancel() {
Guard<Mutex> g(lock);
canceled = true;
bool isCanceled() {
Guard<Mutex> g(lock);
return canceled;
class Accessor : public Runnable {
int id;
CountedPtr<ThreadLocalVariables> tlv;
Accessor(CountedPtr<ThreadLocalVariables>& tl, int idn)
: id(idn), tlv(tl) {}
void run() {
while(!tlv->isCanceled()) {
cout << *this << endl;
friend ostream&
operator<<(ostream& os, Accessor& a) {
return os << "#" << << ": " << a.tlv->get();
int main() {
cout << "Press <Enter> to quit" << endl;
try {
tlv(new ThreadLocalVariables);
const int SZ = 5;
ThreadedExecutor executor;
for(int i = 0; i < SZ; i++)
executor.execute(new Accessor(tlv, i));
tlv->cancel(); // All Accessors will quit
} catch(Synchronization_Exception& e) {
cerr << e.what() << endl;
} ///:~

When you create a ThreadLocal object by instantiating the template, you are only able to access the contents of the object using the get( ) and set( ) member functions. The get( ) function returns a copy of the object that is associated with that thread, and set( ) inserts its argument into the object stored for that thread, returning the old object that was in storage. You can see this is use in increment( ) and get( ) in ThreadLocalVariables.

Since tlv is shared by multiple Accessor objects, it is written as Cancelable so that the Accessors can be signaled when we want to shut the system down.

When you run this program, you ll see evidence that the individual threads are each allocated their own storage.

Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire