I am using boost io_service to run methods asynchronously:
void my_class::completion_handler()
{
...
}
m_io_service.post(boost::bind(&my_class::completion_handler, this));
I would like to use lambda expression instead of boost::bind (see below) in order to avoid creating method for each handler but I am using a C++ compiler that does not support C++11 fully:
m_io_service.post([this](){ ... });
Is it possible to have the same behavior by using phoenix lambda ?
Thank you.
Yes that's possible.
Most notable difference is the placeholders (don't use std::place_holders::_1, _2... but boost::phoenix::arg_names::arg1, arg2...).
However, simply replacing boost::bind with std::bind, boost::lambda::bind or boost::phoenix::bind is ultimately useless of course.
Instead you could use Phoenix actors to compose "lambdas", like e.g.
namespace phx = boost::phoenix;
boost::mutex mx;
boost::condition_variable cv;
boost::unique_lock<boost::mutex> lk(mx);
vc.wait(lk, phx::ref(m_queue_size) > 0);
Member invocations are tricky in that respect.
The good news is that Phoenix comes with implementations of many STL operations like size(), empty(), push_back() etc.
Similar use of Phoenix in this queue implementation: Boost group_threads Maximal number of parallel thread and e.g. asio::io_service and thread_group lifecycle issue).
boost::fusion::function<>
You can adapt free functions with BOOST_PHOENIX_ADAPT_FUNCTION and function objects with BOOST_PHOENIX_ADAPT_CALLABLE. However in the latter case it's probably more elegant to use boost::fusion::function<>:
struct MyType {
MyType()
: complete_(complete_f { this })
{ }
void doSomething() { }
private:
struct complete_f {
MyType* _this;
void operator()() const {
// do something with _this, e.g
this->doSomething();
}
};
boost::phoenix::function<complete_f> complete_;
};
Related
I am exploring and trying to learn C++ Coroutines (added in C++20). An SDK I am using has asynchronous API calls which all take a callback, the callbacks are invoked on some background thread managed by the SDK.
namespace third_party {
bool api_call(const std::string& some_parameter, const std::function<void(std::error_code)>& callback);
} // namespace third_party
I would like wrap this API call into something which can be awaited instead:
namespace my_third_party_sdk_wrapper {
cppcoro::task<std::error_code> api_call(const std::string& some_parameter);
cppcoro::task<std::error_code> api_call(const std::string& some_parameter, cppcoro::cancellation_token token);
} // namespace my_third_party_sdk_wrapper
I am considering using the cppcoro lib but that isn't a requirement unless the implementation of the wrapper gets much simpler by doing so.
The problem is I cannot figure out how implement the wrapper.
There's a really good article by Raymond Chen, you can find it here.
In your case, you can do something like this.
namespace my_third_party_async_sdk_wrapper
{
auto api_call_async(const std::string& some_parameter)
{
struct awaiter : public std::experimental::suspend_always
{
awaiter(const std::string ¶meter)
:parameter_(parmeter) {}
bool await_ready() { return true; }
void await_suspend(std::experimental::coroutine_handle<> handle)
{
// use your third party lib call directly here.
api_call(parameter_, [](std::error_code ec)
{
// call the handle to resume the coroutine
handle();
}
}
};
return awaiter(some_parameter);
}
}
This should do what you want.
a simple (running) example on cppreference.com, "switch_to_new_thread": https://en.cppreference.com/w/cpp/language/coroutines
open are two problems:
passing a parameter to the callback
chaining the callbacks down the callstack
this code shows a solution https://wandbox.org/permlink/OGjmtFWsgjnn3GxX
look for latest version here: https://github.com/andreaspfaffenbichler/Continuation/blob/master/Continuation.cpp
Intro: For synchronization, C# offers the System.Threading.Monitorclass, offering thread synchronization routines such as Enter(), Exit(), TryEnter() and alike.
Furthermore, there is the lock statement that makes sure a lock gets destroyed when a critical code block is left, either by normal execution flow or by an exception:
private static readonly obj = new Object();
lock(obj) {
...
}
Problem: In C++, for this purpose, we got the RAII wrappers std::lock_guard and std::unique_lock that are not applied to Monitor classes but to types fulfilling the Lockable concept. However, I consider this approach syntactically weaker than the way C# implemented it for several reasons:
You pollute the local scope with a variable name that cannot be reused. This can be countered by adding new scopes like
{
std::unique_lock<std::mutex> lck{ mtx };
...
}
But I find this notation rather awkward-looking. What troubles me even more that this is valid C++:
std::unique_lock<std::mutex>{ mtx ]; // note there is no name to the lock!
...
So by forgetting to give a proper name to the lock guard, this statement will be interpreted as a variable declaration named "mtx" of type std::unique_lock<std::mutex>, without having anything locked!
I want to implement something like the lock statement from C# in C++. In C++17, this can be accomplished very easily:
#define LOCK(mutex) if(std::lock_guard<decltype(mutex)> My_Lock_{ mutex }; true)
std::mutex mtx;
LOCK(mtx) {
...
}
Q: How can I implement this in C++11/14?
Putting aside the "should you do this", here's how:
While it's not quite the same, since it requires a semi-colon, it's near enough that I feel I may present it. This pure C++14 solution basically just defines the macro to start a lambda which is immediately executed:
template<typename MTX>
struct my_lock_holder {
MTX& mtx;
my_lock_holder(MTX& m) : mtx{m} {}
};
template<typename MTX, typename F>
void operator+(my_lock_holder<MTX>&& h, F&& f) {
std::lock_guard<MTX> guard{h.mtx};
std::forward<F>(f)();
}
#define LOCK(mtx) my_lock_holder<decltype(mtx)>{mtx} + [&]
The my_lock_holder just nabs the mutex reference for later, and allows us to overload operator+. The idea is that the operator creates the guard and execute the lambda. As you can see the macro defines a default reference capture, so that lambda will be able to reference anything in the enclosing scope. Then it's pretty much straight forward:
std::mutex mtx;
LOCK(mtx) {
}; // Note the semi-colon
And you can see it build live.
Inspired by StoryTeller's great idea, I think I found a viable solution myself, despite being somewhat a "hack":
template <typename T>
struct Weird_lock final : private std::lock_guard<T> {
bool flip;
Weird_lock(T& m) : std::lock_guard<T>{ m }, flip{ true } { }
operator bool() noexcept {
bool old = flip;
flip = false;
return old;
}
};
#define LOCK(mutex) for(Weird_lock<decltype(mutex)> W__l__{ mutex }; W__l__;)
The good thing is that it doesn't need a semicolon in the end. The bad is the need for an additional bool, but from what I see in godbolt.org, the compiler optimizes this out anyways.
I suggest you do:
#define UNIQUE_NAME(name) name##__COUNTER__
#define LOCK(mutex) std::lock_guard<decltype(mutex)> UNIQUE_NAME(My_Lock){ mutex };
Using the
COUNTER preprocessor symbol will generate a unique variable name that you simply don't care about.
When dealing with concurrency problems I often use std::unique_lock<std::mutex> and std::lock_guard<std::mutex>, no problem with both.
I also extended std::mutex to be able to use it as follow:
mutex.protect([](){
// my protected code here
}) ;
It locks the mutex and releases it around the lambda call.
Is such a similar behavior already implemented inside boost or the standard library?
Boost Thread has this: http://www.boost.org/doc/libs/1_58_0/doc/html/thread/synchronization.html#thread.synchronization.with_lock_guard
You can use it like you'd expect:
std::mutex mx;
boost::with_lock_guard(mx, []{
// protected stuff
});
It even supports the usual INVOKE semantics:
int foo(int,double) { return 42; }
// ...
int answer = boost::with_lock_guard(mx, foo, 3, 3.14);
Manual Standard Library Only implementation
You can easily add a thing like this yourself:
template <typename M, typename F, typename... Args>
auto my_with_lock_guard(M& mx, F&& f, Args&&... args) {
std::lock_guard<M> lk(mx);
return std::forward<F>(f)(std::forward<Args>(args)...);
}
If the standard ever adopts a proposal like this you can easily swap it out.
If all you want to do is protect code in a smaller scope inside a function you don't need to extend mutex by writing your own protect function. You can just do as below, use curly braces to create a local scope, and when the scope is exited the mutex will automatically be unlocked in an exception safe way.
double process_func() {
// do some stuff
{ //start a new scope block
std::lock_guard<my_mutex> g; // mutex is locked here.
[]() { } // your lambda that needs to be protected
} // Mutex is released here.
// do more stuff
}
Of course this has the disadvantage over your custom function that,
it is difficult to maintain. Someone can come later and inject more code without knowing what they are doing.
I have to use a library that makes database calls which are not thread-safe. Also I occasionally have to load larger amounts of data in a background thread.
It is hard to say which library functions actually access the DB, so I think the safest approach for me is to protect every library call with a lock.
Let's say I have a library object:
dbLib::SomeObject someObject;
Right now I can do something like this:
dbLib::ErrorCode errorCode = 0;
std::list<dbLib::Item> items;
{
DbLock dbLock;
errorCode = someObject.someFunction(&items);
} // dbLock goes out of scope
I would like to simplify that to something like this (or even simpler):
dbLib::ErrorCode errorCode =
protectedCall(someObject, &dbLib::SomeObject::someFunction(&items));
The main advantage of this would be that I won't have to duplicate the interface of dbLib::SomeObject in order to protect each call with a lock.
I'm pretty sure that this is a common pattern/idiom but I don't know its name or what keywords to search for. (Looking at http://www.vincehuston.org/dp/gof_intents.html I think, it's more an idiom than a pattern).
Where do I have to look for more information?
You could make protectedCall a template function that takes a functor without arguments (meaning you'd bind the arguments at the call-site), and then creates a scoped lock, calls the functor, and returns its value. For example something like:
template <typename Ret>
Ret protectedCall(boost::function<Ret ()> func)
{
DbLock lock;
return func();
}
You'd then call it like this:
dbLib::ErrorCode errorCode = protectedCall(boost::bind(&dbLib::SomeObject::someFunction, &items));
EDIT. In case you're using C++0x, you can use std::function and std::bind instead of the boost equivalents.
In C++0x, you can implement some form of decorators:
template <typename F>
auto protect(F&& f) -> decltype(f())
{
DbLock lock;
return f();
}
usage:
dbLib::ErrorCode errorCode = protect([&]()
{
return someObject.someFunction(&items);
});
From your description this would seem a job for Decorator Pattern.
However, especially in the case of resources, I wouldn't recommend using it.
The reason is that in general these functions tend to scale badly, require higher level (less finegrained) locking for consistency, or return references to internal structures that require the lock to stay locked until all information is read.
Think, e.g. about a DB function that calls a stored procedure that returns a BLOB (stream) or a ref cursor: the streams should not be read outside of the lock.
What to do?
I recommend instead to use the Facade Pattern. Instead of composing your operations directly in terms of DB calls, implement a facade that uses the DB layer; This layer could then manage the locking at exactly the required level (and optimize where needed: you could have the facade be implemented as a thread-local Singleton, and use separate resources, obviating the need for locks, e.g.)
The simplest (and still straightforward) solution might be to write a function which returns a proxy for the object. The proxy does the locking and overloads -> to allow calling the object. Here is an example:
#include <cstdio>
template<class T>
class call_proxy
{
T &item;
public:
call_proxy(T &t) : item(t) { puts("LOCK"); }
T *operator -> () { return &item; }
~call_proxy() { puts("UNLOCK"); }
};
template<class T>
call_proxy<T> protect(T &t)
{
return call_proxy<T>(t);
}
Here's how to use it:
class Intf
{
public:
void function()
{
puts("foo");
}
};
int main()
{
Intf a;
protect(a)->function();
}
The output should be:
LOCK
foo
UNLOCK
If you want the lock to happen before the evaluation of the arguments, then can use this macro:
#define PCALL(X,APPL) (protect(X), (X).APPL)
PCALL(x,x.function());
This evaluates x twice though.
This article by Andrei Alexandrescu has a pretty interesting article how to create this kind of thin wrapper and combine it with dreaded volatile keyword for thread safety.
Mutex locking is a similar problem. It asked for help here: Need some feedback on how to make a class "thread-safe"
The solution I came up with was a wrapper class that prevents access to the protected object. Access can be obtained via an "accessor" class. The accessor will lock the mutex in its constructor and unlock it on destruction. See the "ThreadSafe" and "Locker" classes in Threading.h for more details.
I'm working on a C++ project that needs to run many jobs in a threadpool. The jobs are failure-prone, which means that I need to know how each job terminated after it completes. Being a Java programmer for the most part, I like the idea of using "futures" or a similar paradigm, akin to the various classes in Java's util.concurrent package.
I have two questions: first, does something like this already exist for C++ (I haven't found anything in Boost, but maybe I'm not looking hard enough); and second, is this even a sane idea for C++?
I found a brief example of what I'm trying to accomplish here:
http://www.boostcookbook.com/Recipe:/1234841
Does this approach make sense?
Futures are both present in the upcoming standard (C++0x) and inside boost. Note that while the main name future is the same, you will need to read into the documentation to locate other types and to understand the semantics. I don't know Java futures, so I cannot tell you where they differ, if they do.
The library in boost was written by Anthony Williams, that I believe was also involved in the definition of that part of the standard. He has also written C++ Concurrency in Action, that includes a good description of futures, tasks, promises and related objects. His company also sells a complete and up to implementation of the C++0x threading libraries, if you are interested.
Boost has futures and other threading tools implemented.
Note that when you call the get() method on a boost::unique_future it will re-throw any exception that might have been stored inside it during asynchronous execution.
I would suggest you do something like:
#pragma once
#include <tbb/concurrent_queue.h>
#include <boost/thread.hpp>
#include <boost/noncopyable.hpp>
#include <functional>
namespace internal
{
template<typename T>
struct move_on_copy
{
move_on_copy(const move_on_copy<T>& other) : value(std::move(other.value)){}
move_on_copy(T&& value) : value(std::move(value)){}
mutable T value;
};
template<typename T>
move_on_copy<T> make_move_on_copy(T&& value)
{
return move_on_copy<T>(std::move(value));
}
}
class executor : boost::noncopyable
{
boost::thread thread_;
tbb::concurrent_bounded_queue<std::function<void()>> execution_queue_;
template<typename Func>
auto create_task(Func&& func) -> boost::packaged_task<decltype(func())> // noexcept
{
typedef boost::packaged_task<decltype(func())> task_type;
auto task = task_type(std::forward<Func>(func));
task.set_wait_callback(std::function<void(task_type&)>([=](task_type& my_task) // The std::function wrapper is required in order to add ::result_type to functor class.
{
try
{
if(boost::this_thread::get_id() == thread_.get_id()) // Avoids potential deadlock.
my_task();
}
catch(boost::task_already_started&){}
}));
return std::move(task);
}
public:
explicit executor() // noexcept
{
thread_ = boost::thread([this]{run();});
}
~executor() // noexcept
{
execution_queue_.push(nullptr); // Wake the execution thread.
thread_.join();
}
template<typename Func>
auto begin_invoke(Func&& func) -> boost::unique_future<decltype(func())> // noexcept
{
// Create a move on copy adaptor to avoid copying the functor into the queue, tbb::concurrent_queue does not support move semantics.
auto task_adaptor = internal::make_move_on_copy(create_task(func));
auto future = task_adaptor.value.get_future();
execution_queue_.push([=]
{
try{task_adaptor.value();}
catch(boost::task_already_started&){}
});
return std::move(future);
}
template<typename Func>
auto invoke(Func&& func) -> decltype(func()) // noexcept
{
if(boost::this_thread::get_id() == thread_.get_id()) // Avoids potential deadlock.
return func();
return begin_invoke(std::forward<Func>(func), prioriy).get();
}
private:
void run() // noexcept
{
while(true)
{
std::function<void()> func;
execution_queue_.pop(func);
if(!func)
break;
func();
}
}
};
C++ templates are less restrictive than Java Generics so 'Future's could readily be ported with them and thread synchronization primitives. As for existing libraries which support such a mechanism, hopefully someone else knows of one.