@classmethod decorator in C++

Question

I have the following code :

class A{
    
        void A(){
                counter++;
        }
        static int counter;
        int mem;
        public:
        static int get_counter(){

                this.mem = 0;
                return counter;
        }

};

I remember when I want to share instance and static in a function in python, I use @classmethod decorator.How can I implement it in c++?

Answer 1

In C++ there is no decorator syntax, but ways to implement similar features, such as macros and the CRTP.

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To compare with Python's @classmethod, we'll need to think of a usage. As far as I know @classmethod is typically used to implement some sort of "factory pattern". For C++ in such cases, you'll only need a properly defined constructor and a static function that returns an instance of the class.

Forgive me as I didn't get it what you were trying to do by this.mem = 0, I came up with the following usage (explain your case and I'll replace it).

In Python, suppose you want something like:

class FooBar:
    foo: str
    bar: str
    
    def __init__(self, foo_val: str, bar_val: str):
        self.foo = foo_val
        self.bar = bar_val

    @classmethod
    def from_string(cls, data: str):
        tmp = data.split(' ', 1)
        cls.foo, cls.bar = tmp[0], tmp[1]
        return cls

In C++, similar functionality can be achieved by:

#include <string>
#include <utility>

class FooBar {
public:
    std::string foo;
    std::string bar;

    FooBar(std::string foo_val, std::string bar_val):
        foo(std::move(foo_val)),
        bar(std::move(bar_val)) {}

    static FooBar from_string(const std::string& data) {
        auto pos = data.find_first_of(' ');
        return FooBar(
            data.substr(0, pos),
            data.substr(pos + 1, data.length() - pos - 1)
        );
    }
};

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Also, static member functions are available both by ClassName:: and by instance., while the former is preferred.