While learning about objective-c. I came across a single feature of objective-c which I think if moved to c++ would make for a lot less coding.
Consider this objective c code for a class declaration:
// .h file:
@interface classname {
// instance variables
}
+(void)classMethod;
@end
// .m file:
@implementation classname
-privateFunction1{}
-classMethod { [self privateFunction1]; }
@end
While if you would have to do something similar in c++, this is how you would do it:
// .h file:
class classname
{
public:
void classMethod();
private:
void privateFunction();
}
// .cpp file:
void classname::classMethod() { privateFunction(); }
void classname::privateFunction() {}
Even if a function in a C++ class is private, you still have to declare it in the class declaration. why ?
It would be so much better if all the private functions can just placed in the cpp file, and we dont have to put them in the class declaration.
Objective C has the benefit of using @implementation … @end to figure out such methods. But if in c++ if we just declare a function in the .cpp file and such function is not present in the class declaration, the compiler can know that its a private function. Like so:
// .h file:
class sample
{
public:
void doSomething();
private:
int x;
};
// .cpp file:
void sample::doSomethingPrivately()
{
// has access to member data.
x = 7;
}
void sample::doSomething()
{
/// ...
doSomethingPrivately();
}
Robin Burchell
April 9, 2010 at 11:30 pm
There is a solution to this, known as the PIMPL pattern (amongst other things)
Basically, you declare a pointer to a seperate class, and use that to store all your data members/methods you want private/etc. Qt is one big user of this pattern.
For example:
// .h
// forward declaration
class MyFooPrivate;
class MyFoo
{
void myBar();
private:
MyFooPrivate *d_ptr;
};
// cpp
class MyFooPrivate
{
void mooCow()
{
// make a moo noise
}
};
void MyFoo::myBar()
{
d_ptr->mooCow();
// do other stuff
}
digitalSurgeon
April 13, 2010 at 9:38 am
@Robin, I am familiar with that approach ( thanks to Qt ). But I think this could be added to the next C++ language update. Do you see any thing wrong with it ? I mean not declaring private functions in the class declaration.
RFC: Improving Qt’s properties. | Technical Difficulties
July 1, 2010 at 4:40 pm
[...] look like from stone age. Objective-C has some nice features as well, one I mentioned in my previous post, which Qt already solves rather nicely and in a better way as one of the commenter pointed out. [...]
Atif
April 19, 2011 at 3:19 pm
Another way to do just that, is to make the methods file static functions if they are truly private, and not publish them in the header, but at the top of the cpp file.
Of course, these functions are no longer member functions, so public member functions have to pass them the references to class state.