Re: Threaded SmartPtr questions

James Kanze <>
Thu, 7 May 2009 01:36:13 -0700 (PDT)
On May 6, 11:44 pm, dbtouch <> wrote:

I am studying Modern C++ design

I'm not sure of your level, but from what I've seen and heard of
it, I don't think it's exactly for beginners in C++.

and I simplified a threaded
smart pointer implementation as following code. As I execute
it I found that tsp is actually deleted twice. I am trying to
see whether my approach is correct and would like to hear your

There are at least two serious problems with your code.

 #include <iostream>

class Foo {
  void func() {
    std::cout << "Foo::func()" << std::endl;

  Foo(const Foo& f) {
     std::cout << "Foo::Foo(const Foo&)" << std::endl;

  Foo() {
    std::cout << "Foo::Foo()" << std::endl;

  ~Foo() {
    std::cout << "Foo:~Foo()" << std::endl;

class Mutex {

Not important in your example, but a real Mutex will have
identity. Which means that it won't support copy nor
assignment. Whatever you end up designing, you must take this
into account.

  void acquire() {
    std::cout << "Mutex.acquire()" << std::endl;
  void release() {
    std::cout << "Mutex.release()" << std::endl;

template <typename SharedObject, typename SyncPolicy>
class ThreadedSmartPtr {
  SharedObject* m_ptr;
  Mutex lock;

If Mutex isn't copiable, nor is your ThreadedSmartPtr. (That
may or may not be a problem. The smart pointers I've seen which
manage locks all use reference counting to support copy.)

And of course, since the mutex is only known to the single
instance, there's no way it can protect anything not using the
particular instance.

  ThreadedSmartPtr(SharedObject* ptr, SyncPolicy& lck) :
    m_ptr(ptr), lock(lck) {

  ~ThreadedSmartPtr() {

  SharedObject* operator -> () {
    return m_ptr;

template <typename T>
class SmartPtr {
  T *object;

  SmartPtr(T* value) : object(value) {

  T operator->() {
    return *object;

  ~SmartPtr() {
    if (object)
      delete object;

typedef ThreadedSmartPtr<Foo, Mutex> oldSmartPtr;

int main() {
  Foo *ptr = new Foo;
  Mutex m;
  oldSmartPtr tsp(ptr, m);

  SmartPtr<oldSmartPtr> sp(&tsp);

And this will result in undefined behavior. As written, your
SmartPtr can only be initialized with dynamically allocated



James Kanze (GABI Software)
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