是否有C ++等同于Objective-C实例和类方法？

Question

I am learning C++ with Xcode (cocos2d-x).

A regular c++ void method I think is equivalent to an obj-c -void Instance method.

Is there is a c++ equivalent of the obj-c +void Class method?

Thanks

Answer 1

There is no direct equivalent to class methods in C++; however, in many cases C++ static methods can be used in place of class methods.

For example, this Objective-C code:

@interface MyClass : NSObject
{
    int _number;
}
+ (MyClass*)newInstance;
- (void)instanceMethod;
@end

@implementation MyClass
+ (MyClass*)newInstance 
{
    return [[self alloc] init]; 
}

- (void)instanceMethod 
{ 
    _number = 123;
    std::cout << _number;
}
@end

int main(void)
{
    MyClass* foo = [MyClass newInstance];
    [foo instanceMethod];
    return 0;
}

is roughly equivalent to this C++ code:

class MyClass
{
    int _number;

public:
    static MyClass* newInstance();
    void instanceMethod();
};

MyClass* MyClass::newInstance() 
{ 
    return new MyClass(); 
}

void MyClass::instanceMethod() 
{ 
    _number = 123; 
    std::cout << _number;
}

int main(void)
{
    MyClass* foo = MyClass::newInstance();
    foo->instanceMethod();
    return 0;
}

That example also illustrates one of the differences between class methods and static methods.

+newInstance will be inherited by subclasses and will always work correctly (it will always return an instance of a subclass). It can also be overridden by a subclass.

Static methods like MyClass::newInstance() cannot be inherited or overridden. It will always return an instance of MyClass .

So when you're porting code between Objective-C and C++ there are cases when you cannot use static methods in place of class methods. But for most cases, C++ static methods are a fine replacement.

Answer 2

class SomeClass {
public:

    static void someMethod();

};

Answer 3

The equivalent of class methods in C++ would be static functions.

static return_type function_name(parameters);

Just like in Objective-C, in static functions you cannot reference instance variables (since there's no instance), only static variables.

Answer 4

For example.

class A{
public:
static void doIt();
};

Then you can call function doIt with:

void main()
{
A::doIt();
}

Answer 5

Objective-C has implicit metaclasses as explored in " What is a meta-class in Objective-C? " article. The full power of metaclasses is not unleashed, however. It would require multiple inheritance support and other technical decisions, as described in famous " Putting Metaclasses to Work " book. That was probably too complex for Objective-C authors, so they decided to hide metaclasses in Objective-C.

Objective-C targets Objective-C runtime (obj.dll or libobjc.dylib), and there were C++ compilers (DTS C++, Direct-to-SOM C++) targetting SOM the same way. This makes DTS C++ closer to Objective-C than C++ in its design. I managed to get old Windows DTS C++ compiler running on Windows 8 this way:

1. Download VAC 3.5 fixpak 9 from IBM FTP. This fixpak contain many files, so you don't even need to full compiler (I have 3.5.7 distro, but fixpak 9 was big enough to do some tests).
2. Unpack to eg C:\\home\\OCTAGRAM\\DTS
3. Start command line and run subsequent commands there
4. Run: set SOMBASE=C:\\home\\OCTAGRAM\\DTS\\ibmcppw
5. Run: C:\\home\\OCTAGRAM\\DTS\\ibmcppw\\bin\\SOMENV.BAT
6. Run: cd C:\\home\\OCTAGRAM\\DTS\\ibmcppw\\samples\\compiler\\dts
7. Run: nmake clean
8. Run: nmake
9. hhmain.exe and its dll are in different directories, so we must make them find each other somehow; since I was doing several experiments, I executed "set PATH=%PATH%;C:\\home\\OCTAGRAM\\DTS\\ibmcppw\\samples\\compiler\\dts\\xhmain\\dtsdll" once, but you can just copy dll near to hhmain.exe
10. Run: hhmain.exe

I've got an output this way:

Local anInfo->x = 5
Local anInfo->_get_x() = 5
Local anInfo->y = A
Local anInfo->_get_y() = B
{An instance of class info at address 0092E318

}

This example does not use metaclasses, it's just for checking DTS C++ compiler.

Metaclasses in SOM are explicit, thus so called "class methods" are no more than "instance methods" of metaclass. Each object belongs to some class, managed by so called class-object created in runtime. Class-object is itself an instance of another class, named metaclass. Developer of a class can specify metaclass constraint via IDL or DTS C++ pragma.

If one needs to call method of known at compile time class, one can just reference class-object and invoke its method just like ordinary object which class-object is in essence. Class-object is being referenced via DLL import/export mechanisms, similar to Objective-C.

If one needs to invoke a method of a class that object is belonging to, one needs to invoke somGetClass() to get class-object, typecast it to metaclass type and invoke desired method. In DTS C++ one should probably not need to do redundant typecasting, but I am not expert in IBM DTS C++ properties.