C++

Wednesday, November 28. 2007

Practicing my template skills

While working on assignments for the Computergraphics course I'm taking this semester I was missing signals and slots. Using Qt for the assignments is not possible (well actually I didn't ask explicitly whether I could use it), so I felt challenged to code the convenient things I'm so used to...

I knew that boost had some implementation of signals and slots that used templates but I had never actually taken a look at it. And I didn't want to, yet. I wanted to think about the problem myself first. Basically I came up with that a signal needs to be an object where as a slot may not have to be an object, but must also work as a member function. After a few iterations I came up with a header that now allows me to do the following:

#include "signal.h"
#include <iostream>

struct A
{
void operator()()
{
std::cout << "A::operator()" << std::endl;
}

void f()
{
std::cout << "A::f" << std::endl;
}
};

struct B
{
void operator()(int x)
{
std::cout << "B::operator() " << x << std::endl;
}

void f(int x)
{
std::cout << "B::f " << x << std::endl;
}
};

void g()
{
std::cout << "g" << std::endl;
}

void g(int x)
{
std::cout << "g " << x << std::endl;
}

int main()
{
A a;
Signal<> s;
s.connect(&a);
s.connect(&a, &A::f);
s.connect(&g);
s();

B b;
Signal<int> s2;
s2.connect(&b);
s2.connect(&b, &B::f);
s2.connect(&g);
s2(5);

return 0;
}

The output of that program is: A::operator() A::f g B::operator() 5 B::f 5 g 5 as you probably expected. Notice that

You can use 0 to 10 types in the template parameter of the Signal class.
You can connect to functions, member functions and objects that have an operator() with the right signature. This allows to connect one signal to another one, like:
Signal<const std::vector<int> &> s1;
Signal<const std::vector<int> &> s2;
s1.connect(&s2);
s2.connect(&someFunction);
s1(std::vector<int>());
This will call someFunction(std::vector()).
Signal will only allow "slot" connections and operator() calls with the correct signature. I needed to make Signal have different member functions depending on the number of its template arguments. Took me some time to realize but this was possible using inheritance.