This is a quick note to chapter 4 of C++ Concurrency in Action.

1. std::thread

In C++11, It’s quite simple to create a separate thread using std::thread. Following code will simply output “hello world” or “world hello”:

2. std::mutex and std::condition_variable

If you need synchronization between threads, there are std::mutex and std::condition_variable. The semantics are the same with that in pthread library. Here’s a simple producer/consumer demo:

3. std::future with std::async()

C++11 also simplifies our work with one-off events with std::future. std::future provides a mechanism to access the result of asynchronous operations. It can be used with std::async(), std::packaged_task and std::promise. Starting with std::async():

std::async() gives two advantages over the direct usage of std::thread. Threads created by it are automatically joined. And we can now have a return value. std::async() decides whether to run the callback function in a separate thread or just in the current thread. But there’s a chance to specify a control flag(launch::async or launch::deferred) to tell the library, what approach we want it to run the callback.

When testing With gcc-4.8, foo() is not called. But with VC++2013, it does output “hello”.

4. std::future with std::packaged_task

With std::async(), we cannot control when our callback function is invoked. That’s what std::packaged_task is designed to deal with. It’s just a wrapper to callables. We can request an associated std::future from it. And when a std::packaged_task is invoked and finished, the associated future will be ready:

In waiter() and waiter2(), future::get() blocks until the associating std::packaged_task completes. You will always get “in pt” before “after f.get()” and “in pt2” before “after f2.get()”. They are synchronized.

5. std::future with std::promise

You may also need to get notified in the middle of a task. std::promise can help you. It works like a lightweight event.

Future and Promise are the two separate sides of an asynchronous operation. std::promise is used by the “producer/writer”, while std::future is used by the “consumer/reader”. The reason it is separated into these two interfaces is to hide the “write/set” functionality from the “consumer/reader”:

Again in waiter() and waiter2(), future::get() blocks until a value or an exception is set into the associating std::promise. So “setting p” is always before “f.get()” and “setting p2” is always before “f2.get()”. They are synchronized.

NOTE: std::future seems to be not correctly implemented in VC++2013. So the last two code snippet do not work with it. But you can try the online VC++2015 compiler(still in preview as this writing), it works.

1. Environment

– windows xp
– gcc-4.4
– boost-1.43

2. auto_ptr

A smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. There’s auto_ptr in C++03 library for general use. But it’s not so easy to deal with it. You may encounter pitfalls or limitations. The main drawback of auto_ptr is that it has the transfer-of-ownership semantic. I just walk through it. Please read comments in code carefully:

3. unique_ptr

To resolve the drawbacks, C++0x deprecates usage of auto_ptr, and unique_ptr is the replacement. unique_ptr makes use of a new C++ langauge feature called rvalue reference which is similar to our current (left) reference (&), but spelled (&&). GCC implemented this feature in 4.3, but unique_ptr is only available begin from 4.4.

What is rvalue?

rvalues are temporaries that evaporate at the end of the full-expression in which they live (“at the semicolon”). For example, 1729, x + y, std::string(“meow”), and x++ are all rvalues.

While, lvalues name objects that persist beyond a single expression. For example, obj, *ptr, ptr[index], and ++x are all lvalues.

NOTE: It’s important to remember: lvalueness versus rvalueness is a property of expressions, not of objects.

We may have another whole post to address the rvalue feature. Now, let’s take a look of the basic usage. Please carefully reading the comments:

One can ONLY make a copy of an rvalue unique_ptr. This confirms no ownership issues occur like that of auto_ptr. Since temporary values cannot be referenced after the current expression, it is impossible for two unique_ptr to refer to a same pointer. You may also noticed the move function. We will also discuss it in a later post.

Some more snippet:

unique_ptr can hold pointers to an array. unique_ptr defines deleters to free memory of its internal pointer. There are pre-defined default_deleter using delete and delete[](array) for general deallocation. You can also define your customized ones. In addition, a void type can be used.

NOTE: To compile the code, you must specify the -std=c++0x flag.

4. shared_ptr

A shared_ptr is used to represent shared ownership; that is, when two pieces of code needs access to some data but neither has exclusive ownership (in the sense of being responsible for destroying the object). A shared_ptr is a kind of counted pointer where the object pointed to is deleted when the use count goes to zero.

Following snippet shows the use count changes when using shared_ptr. The use count changes from 0 to 3, then changes back to 0:

Snippets showing pointer type conversion:

The void type can be used directly without a custom deleter, which is required in unique_ptr. Actually, shared_ptr has already save the exact type info in its constructor. Refer to source code for details :). And static_pointer_cast function is used to convert between pointer types.

Unlike auto_ptr, Since shared_ptr can be shared, it can be used in STL containers:

NOTE: shared_ptr is available in both TR1 and Boost library. You can use either of them, for their interfaces are compatible. In addition, there are dual C++0x and TR1 implementation. The TR1 implementation is considered relatively stable, so is unlikely to change unless bug fixes require it.

5. weak_ptr

weak_ptr objects are used for breaking cycles in data structures. See snippet:

If we use uncomment to use shared_ptr, head is not freed since there still one reference to it when exiting the function. By using weak_ptr, this code works fine.

6. scoped_ptr

scoped_ptr template is a simple solution for simple needs. It supplies a basic “resource acquisition is initialization” facility, without shared-ownership or transfer-of-ownership semantics.

This class is only available in Boost. Since unique_ptr is already there in C++0x, this class may be thought as redundant. Snippet is also simple:

Complete and updated code can be found on google code host here. I use conditional compilation to swith usage between TR1 and Boost implementation in code. Hope you find it useful.

Let clarify some concepts first. What is C++0x? Wikipedia gives some overview here:

C++0x is intended to replace the existing C++ standard, ISO/IEC 14882, which was published in 1998 and updated in 2003. These predecessors are informally but commonly known as C++98 and C++03. The new standard will include several additions to the core language and will extend the C++ standard library, incorporating most of the C++ Technical Report 1 (TR1) libraries — with the exception of the library of mathematical special functions.

Then why it is called C++0x? As Bjarne Stroustrup addressed here:

The aim is for the ‘x’ in C++0x to become ‘9’: C++09, rather than (say) C++0xA (hexadecimal :-).

You may also noticed TR1, also refer here in Wikipedia:

C++ Technical Report 1 (TR1) is the common name for ISO/IEC TR 19768, C++ Library Extensions, which is a document proposing additions to the C++ standard library. The additions include regular expressions, smart pointers, hash tables, and random number generators. TR1 is not a standard itself, but rather a draft document. However, most of its proposals are likely to become part of the next official standard.

You got the relationship? C++0x is the standard adding features to both language and standard library. A large set of TR1 libraries and some additional libraries. For instance, unique_ptr is not defined in TR1, but is included in C++0x.

As of 12 August 2011, the C++0x specification has been approved by the ISO.

Another notable concept is the Boost library. It can be regarded as a portable, easy-to-use extension to the current C++03 standard library. And some libraries like smart pointers, regular expressions have already been included in TR1. You can find license headers regarding the donation of the boost code in libstdc++ source files. While in TR2, some more boost code are to be involved.

TR1 libraries can be accessed using std::tr1 namespace. More info on Wikipedia here:

Various full and partial implementations of TR1 are currently available using the namespace std::tr1. For C++0x they will be moved to namespace std. However, as TR1 features are brought into the C++0x standard library, they are upgraded where appropriate with C++0x language features that were not available in the initial TR1 version. Also, they may be enhanced with features that were possible under C++03, but were not part of the original TR1 specification.

The committee intends to create a second technical report (called TR2) after the standardization of C++0x is complete. Library proposals which are not ready in time for C++0x will be put into TR2 or further technical reports.

The article seems to be a bit too long so far, I decide to give my snippets in a later post.