bind function with return value

[Frankie2017]

I am studying thread-pool with asynchronous I/O. Now, I tried to bind a function with return value. However, it failed. Can anyone please help (the code which starts with "ioService.post")?

Thank you very much.

#include <vector>
#include <ppl.h>
#include <iostream>
#include <chrono>
#include <boost/asio/io_service.hpp>
#include <boost/thread/thread.hpp>
#include <boost/phoenix/phoenix.hpp>
#include <boost/function.hpp>

using namespace std;

typedef vector<double> Vector;
typedef vector<Vector> Matrix;

class myClass {
private:
    int num;
    Matrix a;
    Vector z;

public:
    myClass(int n) :
        num(n), a(num, Vector(2)) {}

    Vector subFun(int n) {
        Vector b(2, 0);

        b[0] = n;
        b[1] = n+num;
        //body: to be implemented

        return b;
    }

    void mainFun() {

        Matrix z;
        Vector w;
        boost::asio::io_service ioService;
        boost::thread_group threadpool;

        boost::asio::io_service::work work(ioService);

        for (int i = 0; i < std::thread::hardware_concurrency(); i++) {
            threadpool.create_thread(
                boost::bind(&boost::asio::io_service::run, &ioService)
            );
        }

        for (int i = 0; i < num; i++) {
            ioService.post(boost::phoenix::bind(&myClass::z, this) = 
                 boost::phoenix::bind(&myClass::subFun, this, i));//OK

            ioService.post(boost::phoenix::bind(&myClass::a[i], this) = 
                 boost::phoenix::bind(&myClass::subFun, this, i));//FAIL

            ioService.post(boost::phoenix::bind(w) = 
                 boost::phoenix::bind(&myClass::subFun, this, i));
                 //local variable w -->FAIL
        }

        ioService.stop();

        threadpool.join_all();
    }

    Matrix getA() {
        return a;
        }
    };

[zoran404]

Wrap the function that returns a value in a function that instead takes a reference and assigns the return value to that.

[Frankie2017]

Sorry, I cannot understand what you mean. Can you please illustrate your idea by code? Thanks.

[Cubbi]

boost.phoenix is great, but C++ can do this too:

            ioService.post([this,i]{this->z = subFun(i);});
            ioService.post([this,i]{a[i] = subFun(i);});
            ioService.post([this,i,&w]{w = subFun(i);});

[Frankie2017]

Thank you very much.

You use lambda function instead of phoenix bind function. It works. In the capture list, "this" is used because subFun is a member function of myClass, is it right?

[Cubbi]

In the capture list, "this" is used because subFun is a member function of myClass, is it right?

yes (and also because a is a member of myClass), I like to be explicit about capture of the this pointer, because it is sometimes counter-intuitive. See also Core Guidelines: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f54-if-you-capture-this-capture-all-variables-explicitly-no-default-capture