C++ lambda won't call the destructor on members captured by value

Question

I've been badly bitten by this strange behavior of lambdas under XCode today - after trying to trace several memory leaks in iOS around the code, I've narrowed it down to this (and similar) snippet(s) where I assign the ownership of something to a deferred task using a shared pointer:

void DBStorage::dispose(std::shared_ptr<DataChunk>& dc)
{
    backgroundQueue.queueTask([=]() {
        assert( dc.use_count() == 1 );

        if (dc->isDirty()) {
            //store to disk
        }
    });
}

(Note that the shared pointer's use count is always 1 when the lambda is run)

After execution, this task is null-ified with pendingJob = nullptr; which I expected to call the destructor of all the captured-by-value objects, and consequently DataChunk's destructor. However, it looks like that under XCode/LLVM lc's destructor is never called; calling its dtor explicitly, using mutable, and deleting the std::function with a simple delete didn't work either.

Is this standard behavior? I can of course manually call dc.reset() and it works as expected, but this quite makes the point of using a shared pointer moot.

---

Solution Apparently, it is a known gcc bug.

---

Contrib

Stand-alone sample with output from Xcode 5.0.2/clang 3.3

#include <iostream>
#include <memory>

void fnRef(std::shared_ptr<int>& ptr)
{
    auto lambda = [=]() { std::cout << ptr.use_count() << ':' << __PRETTY_FUNCTION__ << '\n'; };
    lambda();
}

void fnVal(std::shared_ptr<int> ptr)
{
    auto lambda = [=]() { std::cout << ptr.use_count() << ':' << __PRETTY_FUNCTION__ << '\n'; };
    lambda();
}

int main()
{
    std::shared_ptr<int> ptr(new int);
    for (int i=0; i<10; ++i)
        fnVal(ptr);
    std::cout << '\n';

    for (int i=0; i<10; ++i)
        fnRef(ptr);

    return 0;
}

LLVM/GCC Output

3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const
3:void fnVal(std::shared_ptr<int>)::<anonymous class>::operator()() const

2:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
3:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
4:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
5:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
6:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
7:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
8:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
9:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
10:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const
11:void fnRef(std::shared_ptr<int> &)::<anonymous class>::operator()() const

IDEOne.com Output for same code

3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
3:fnVal(std::shared_ptr<int>)::__lambda1
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0
2:fnRef(std::shared_ptr<int>&)::__lambda0

Visual Studio 2013 Output

3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()
3:fnVal::<lambda_67137a3f93ee478c018cc7068004c9fd>::operator ()

2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()
2:fnRef::<lambda_70f241d4201227663d23c74be170d302>::operator ()

Answer 1

As noted by @DaveS this could be a known gcc bug -- captured references are stored as references.

A good rule of thumb when working with stored lambdas is to avoid =, as stored state should be treated with care.

void DBStorage::dispose(std::shared_ptr<DataChunk>& dc)
{
  std::shared_ptr<DataChunk> data_to_store = dc;
  backgroundQueue.queueTask([data_to_store]() { // maybe add `,this` to the capture list
    assert( data_to_store.use_count() == 1 );
    if (data_to_store->isDirty()) {
      //store to disk
    }
  });
}

or:

void DBStorage::dispose(std::shared_ptr<DataChunk> data_to_store)
{
  backgroundQueue.queueTask([data_to_store]() { // maybe add `,this` to the capture list
    assert( data_to_store.use_count() == 1 );
    if (data_to_store->isDirty()) {
      //store to disk
    }
  });
}

as a second bit of unsolicited advice, std::functions are not lambdas, and calling one theLambda is misleading.