Re: dynamic function definition
cesco wrote:
Hi,
I need to define the body of a function in the constructor of a class.
For example, the class has a member function Log (that is the public
interface of the class), but I want the body of this function to be
defined as something or as something else in the constructor of the
class:
class MyClass
{
public:
void Log(const int& id, const int& value);
private:
void LogON(const int& id, const int& value);
void LogOFF();
};
The typical C++ approach to this problem would use class inheritance
and virtual methods to implement the necessary dynamic behavior. The
only complication in this particular situation is that the dynamic
behavior of the object is not determined until the object is already
under construction - which rules out implementing the dynamic behavior
in the class of the object itself (the MyClass class in this case).
The usual way for a MyClass class object to behave dynamically after
its construction, is to implement the dynamic behavior in an
"implementation" class object that is stored as a MyClass data member.
Then MyClass effectively implements dynamic behavior by fowarding
static calls to this dynamic data member. The code below shows how this
arrangement might look in practice:
#include <cstdlib>
// declare an abstract base class for the implementation class
class LogImplementation
{
public:
virtual ~LogImplementation() {}
virtual void Log( const int& id,
const int& value) = 0;
};
// a concrete log implementation
class LogOnImplementation : public LogImplementation
{
public:
virtual void Log(const int& id, const int& value)
{
// LogON implementation goes here
}
};
class LogOffImplementation : public LogImplementation
{
public:
virtual void Log(const int& id, const int& value)
{
// LogOFF implementation goes here
}
};
// the MyClass object provides the client interface for logging
class MyClass
{
public:
MyClass();
// the Log method is forwarded to the implementation
void Log(const int& id, const int& value)
{
mImplementation->Log(id, value);
}
private:
LogImplementation *mImplementation;
};
// MyClass constructor selects its dynamic behavior
// by allocating the appropriate implementation data member
MyClass::MyClass()
: mImplementation(NULL)
{
// select the desired behavior here
mImplementation = new LogOnImplementation;
// or
mImplementation = new LogOffImplementation;
}
Greg
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