Introduction
Last week, I had a mission adding a callback function to the system. At first, I thought that’s quite easy so I had the implementation as below, but it ran into an error…
Class A is a listener that needs to execute a callback function when A specific moment and the callback will be set by class B.
Class B has a function cal to be executed and it will register the function to A so that it can be carried out when needed. In cal function, it has to sum the arguments and the value which is calculated by doTrick().
First implementation
Header file:
class A
{
public:
A();
function<int(int, int)> callbackFunc;
void setCB(function<int(int, int)>);
int doCB(int, int);
};
class B{
public:
B(A*);
int doTrick();
static int cal(int, int);
};Cpp file:
A::A()
{
}
void A::setCB(function<int(int, int)> callback)
{
callbackFunc = callback;
}
int A::doCB(int num1, int num2)
{
return callbackFunc(num1, num2);
}
B::B(A *a)
{
a->setCB(cal);
}
int B::doTrick()
{
// Do a trick here.
return 1;
}
int B::cal(int num1, int num2)
{
doTrick();
return num1 + num2;
}main.cpp
int main()
{
A a = A();
B b = B(&a);
// At a certain moment, object a would execute the callback.
cout << a.doCB(3,5);
return 0;
}Error
main.cpp:42:13: error: cannot call member function ‘int B::doTrick()’ without object
doTrick();Explanation
I believe that we all know what callback is so let’s skip it. As the error shows, we cannot call the member function. But why?
That’s because cal is a static function, it cannot access the B class members and functions. And how could we solve it?
Solution
The great Google always give us an answer.
In my opinion, the cal function needs an instance to access the B class’s member functions. Fortunately, We can use std::bind to wrap function so as it can complete registering a callback from a class member.
Solution implementation
Header file:
Please be aware that the cal function is not a static function anymore.
class A
{
public:
A();
function<int(int, int)> callbackFunc;
void setCB(function<int(int, int)>);
int doCB(int, int);
};
class B{
public:
B(A*);
int doTrick();
static int cal(int, int);
};Cpp file:
A::A()
{
}
void A::setCB(function<int(int, int)> callback)
{
callbackFunc = callback;
}
int A::doCB(int num1, int num2)
{
return callbackFunc(num1, num2);
}
B::B(A *a)
{
// Warp the class menber function as a new one.
auto temp = bind(&B::cal, this, placeholders::_1, placeholders::_2);
a->setCB(temp);
}
int B::doTrick()
{
// Do a trick here.
return 1;
}
int B::cal(int num1, int num2)
{
return doTrick() + num1 + num2;
}If you like this post, you can buy me a coffee! :)
