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How to: Provide Work Functions to the call and transformer Classes

This topic illustrates several ways to provide work functions to the concurrency::call and concurrency::transformer classes.

The first example shows how to pass a lambda expression to a call object. The second example shows how to pass a function object to a call object. The third example shows how to bind a class method to a call object.

For illustration, every example in this topic uses the call class. For an example that uses the transformer class, see How to: Use transformer in a Data Pipeline.

The following example shows a common way to use the call class. This example passes a lambda function to the call constructor.

// call-lambda.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Stores the result of the computation.
   single_assignment<int> result;

   // Pass a lambda function to a call object that computes the square 
   // of its input and then sends the result to the message buffer.
   call<int> c([&](int n) {
      send(result, n * n);
   });

   // Send a message to the call object and print the result.
   send(c, 13);
   wcout << L"13 squared is " << receive(result) << L'.' << endl;
}

This example produces the following output.

13 squared is 169.

The following example resembles the previous one, except that it uses the call class together with a function object (functor).

// call-functor.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

// Functor class that computes the square of its input. 
class square
{
public:
   explicit square(ITarget<int>& target)
      : _target(target)
   {
   }

   // Function call operator for the functor class. 
   void operator()(int n)
   {
      send(_target, n * n);
   }

private:
   ITarget<int>& _target;
};

int wmain()
{
   // Stores the result of the computation.
   single_assignment<int> result;

   // Pass a function object to the call constructor.
   square s(result);
   call<int> c(s);

   // Send a message to the call object and print the result.
   send(c, 13);
   wcout << L"13 squared is " << receive(result) << L'.' << endl;
}

The following example resembles the previous one, except that it uses the std::bind1st and std::mem_fun functions to bind a call object to a class method.

Use this technique if you have to bind a call or transformer object to a specific class method instead of the function call operator, operator().

// call-method.cpp 
// compile with: /EHsc
#include <agents.h>
#include <functional>
#include <iostream>

using namespace concurrency;
using namespace std;

// Class that computes the square of its input. 
class square
{
public:
   explicit square(ITarget<int>& target)
      : _target(target)
   {
   }

   // Method that computes the square of its input. 
   void square_value(int n)
   {
      send(_target, n * n);
   }

private:
   ITarget<int>& _target;
};

int wmain()
{
   // Stores the result of the computation.
   single_assignment<int> result;

   // Bind a class method to a call object.
   square s(result);
   call<int> c(bind1st(mem_fun(&square::square_value), &s));

   // Send a message to the call object and print the result.
   send(c, 13);
   wcout << L"13 squared is " << receive(result) << L'.' << endl;
}

You can also assign the result of the bind1st function to a std::function object or use the auto keyword, as shown in the following example.

// Assign to a function object.
function<void(int)> f1 = bind1st(mem_fun(&square::square_value), &s);
call<int> c1(f1);

// Alternatively, use the auto keyword to have the compiler deduce the type.
auto f2 = bind1st(mem_fun(&square::square_value), &s);
call<int> c2(f2);

Copy the example code and paste it in a Visual Studio project, or paste it in a file that is named call.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe /EHsc call.cpp

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