Template Template Method

I recently had to write a bunch of C++ functions that shared a common pattern:

1) acquire a resource lock
2) TRY
3) do some work
4) CATCH exceptions of interest, and log them
5) release resource lock

I was eager to implement the "do some work" bits but did not want to bore myself silly by repeating the steps 1, 2, 4, 5 in each function (about thirty or so of them.) Now, you may recognize the problem, it is what the Template Method Design Pattern solves: "avoid duplication in the code: the general workflow structure is implemented once in the abstract class's algorithm, and necessary variations are implemented in each of the subclasses."

I could not however apply the Template Method pattern "as is" because my functions did not share a common signature. So wrapping them with a non-virtual function and then implement the "do some work" as virtual methods would not work in my case.

One alternative was to code steps 1 and 2 above as a BEGIN_CALL macro, wrap up steps 4 and 5 into a END_CALL and decorate each of my methods with these archaic C-isms. The approach would indeed work for C, but it is utterly indecorous in C++.

The spirit of the Template Method pattern can be preserved very elegantly by making the template method a C++ template method, and wrap the variant "do some work" bits into a lambda block.

The code sample below illustrates the idea (I added some mock objects to give more context).

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

// Mock a resource that needs exclusive locking
//
// -- think CComCriticalSection for example:
// http://msdn.microsoft.com/en-us/library/04tsf4b5(v=vs.80).aspx
class Resource
{
public:
void Lock() { cout << "Resource locked." << endl; }
void Unlock() { cout << "Resource unlocked." << endl; }
};

// Generic stack-based lock. Works with any resource
// that implements Lock() and Unlock() methods.
// See:
// http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization
template<typename T>
class Lock
{
Lock(const Lock&);              // non-copyable
Lock& operator=(const Lock&);   // non-assignable

T&  m_resource;                 // Lock MUST NOT outlive resource

public:
explicit Lock( T& resource ) : m_resource( resource )
{
    m_resource.Lock( );
}

~Lock( )
{
    m_resource.Unlock( );
}
};

/////////////////////////////////////////////////////////////////////////////
// Template Method Wrapper for an arbitrary lambda block:
//  1) lock a resource
//  2) log exceptions
// This is a variant of the Template Method Design Pattern
// -- implemented as a C++ template method.
template<typename F>
auto Execute(Resource& r, F f) -> decltype(f())
{
Lock<Resource> lock(r);
try
{
    return f();
}
catch (const exception& e)
{
    // log error
    clog << e.what() << endl;
}
typedef decltype(f()) result_type;
return result_type();
}
/////////////////////////////////////////////////////////////////////////////

// Usage example:

static Resource globalResource;


int f (int i)
{
return Execute(globalResource, [&]()->int
{
    return i + 1;
});
}

string g (unsigned j)
{
return Execute(globalResource, [&]()->string
{
    ostringstream ss;
    ss << '[' << j << ']';
    return ss.str();
});
}

int main()
{
cout << f(41) << endl;
cout << g(42) << endl;

return 0;
}

Update: to compile the code above using GCC you may need to specify "-std=c++0x" on the command line.

One thing that I grappled with for a bit was how to make the Execute template function figure out the return type of the wrapped lambda. After pinging Andrei Alexandrescu at Facebook (or is it "on Facebook"? No matter -- my English as a second language works either way, because Andrei does work for Facebook) and some googling around, I found the magic incantation: decltype(f()).