QtConcurrent and boost::bind

[TheShow]

I'm having trouble using QtConcurrent::map with boost::bind. The Qt documentation gives a brief example using a QImage, but we need to do something a little different. After searching and trying many different things, I thought I'd start with the most basic example.

#include <boost/bind.hpp>
#include <qtconcurrentmap.h>
 
void doSomething(int value)
{
    // Want to open a file corresponding to a QString here,
    // but how does it get passed in?
}
 
int main(int argc, char *argv[])
{
    QStringList filenames;
    for (int i=0; i<100; i++)
    {
        QString filename("f");
        filename.append(QString::number(i));
        filename.append(".dat");
        filenames << filename;
    }
    QFuture<void> future = QtConcurrent::map(filenames, boost::bind(doSomething, 50));
 
    return 0;
}

#include <boost/bind.hpp>
#include <qtconcurrentmap.h>

void doSomething(int value)
{
    // Want to open a file corresponding to a QString here,
    // but how does it get passed in?
}

int main(int argc, char *argv[])
{
    QStringList filenames;
    for (int i=0; i<100; i++)
    {
        QString filename("f");
        filename.append(QString::number(i));
        filename.append(".dat");
        filenames << filename;
    }
    QFuture<void> future = QtConcurrent::map(filenames, boost::bind(doSomething, 50));

    return 0;
}

To copy to clipboard, switch view to plain text mode 

This passes in the integer 50 to the doSomething function.

The Qt documentation gives an example that doesn't use boost:bind where it takes each item in the Sequence and calls a function:

void scale(QImage &image)
 {
     image = image.scaled(100, 100);
 }
 
 QList<QImage> images = ...;
 QFuture<void> future = QtConcurrent::map(images, scale);

void scale(QImage &image)
 {
     image = image.scaled(100, 100);
 }

 QList<QImage> images = ...;
 QFuture<void> future = QtConcurrent::map(images, scale);

To copy to clipboard, switch view to plain text mode 

In this case, scale is called with the object in the images list (a QImage). In order to pass additional arguments, the documentation says we can combine QtConcurrent::map with boost::bind.

Can somebody tell me how to implement a function that includes the object in the list (QString in my case) with additional parameters?

I wanted this to work:

void doSomething(QString filename, int value)
{
}

void doSomething(QString filename, int value)
{
}

To copy to clipboard, switch view to plain text mode 

but of course it doesn't even compile, indicating that

`void (*)(QString, int)' is not a class, struct, or union type

[fanat9]

Well...

Actually, return from boost::bind is a function object(functor) and QtConcurrent::map() accept such functor's. Boost is heavy, complex library, so I'm personally prefer not to use it when I can. So, take a look on documentation on functor's - maybe you can live without the boost.

But if you have to...
you use boost:bind() to transform your function to a function that takes only one argument and pass result to QtConcurrent::map():

void doSomething(QString &filename, int value)
{
}

void doSomething(QString &filename, int value)
{
}

To copy to clipboard, switch view to plain text mode 

QList<QStrings> list = ...
QFuture<void> future = QtConcurrent::map(list, boost::bind(doSomething, 10));

QList<QStrings> list = ...
QFuture<void> future = QtConcurrent::map(list, boost::bind(doSomething, 10));

To copy to clipboard, switch view to plain text mode