import core.stdc.stdio;

struct A{
    static int e=40;
    struct B{
        int m;
        this(int i){
            printf("%i",e);m=i;
        }
        ~this(){
            e=m;
            printf("%i",e);
        }
    }
    void f(){
        B b=e;e=6;
    }
    void g(){
        printf("%i",e);
    }
}

extern(C)void main(){
    A a;
    a.f();
    a.g();
}

I want B to become the following version similar to 'C++'. What should I do?

struct B{
    int m;
    B(){
        printf("%i",e);m=e;
    }
    ~B(){
        e=m;
        printf("%i",e);
    }
}

Thank you.

On Thursday, 15 June 2023 at 21:48:10 UTC, Ali Çehreli wrote:

    static int e=40;
    struct B{
        int m;
        this(int i){
            printf("%i",e);m=i;
        }
        ~this(){
            e=m;
            printf("%i",e);
        }
//After leaving the scope, restore the original value
    }
// definition,
// usage
    void f(){
        B b=e;e=6;
    }//e Restore to 40.

But I want like follow, but due to the disable this(),I can't:

    void f(){
        B b;e=6;
    }

Because disabled this(), the behavior of this(int i)is inconsistent and cannot achieve similar RAII as C++. What should I do?

On 6/15/23 8:31 PM, zjh wrote:

You cannot have parameter-less constructors in D structs. However, destruction works the same.

Instead, you can use factory functions to initialize.

But there is no way in D to have e.g.:

B b; // runs a constructor

-Steve

On Thursday, 15 June 2023 at 21:48:10 UTC, Ali Çehreli wrote:

I used the class to call the constructor/destructor (destroy), but after they leave the scope, they do not call thedestructor (destroy).
Nested class is because I want to directly use members of external struct, but in reality, I cannot access them directly. Therefore, I only have to use static members,.
extern(C) is to use BetterC,.

The purpose is mainly to complete RAII, completing one part of the work after entering the scope and another part of the work after leaving the scope.

On Friday, 16 June 2023 at 00:35:48 UTC, Steven Schveighoffer wrote:

B b; // runs a constructor

As a C++ user, it is very terrible to simplify RAII without such a method. Why must this() be disabled? Can't there be a constructor? In a constructor, you can complete some work. Without it, how can you simplify RAII?.

On 6/15/23 8:45 PM, zjh wrote:

B b; // runs a constructor

D was not originally designed with RAII. That was added later (D1 structs did not have ctors or dtors).

The benefit of not having parameterless constructors is that default construction is always defined and available at compile time.

What is wrong with using a parameter constructor or factory function? What is the feature you are trying to build that you can with C++, but not with D?

I'm not saying I agree with the limitation, just want to know more.

-Steve

On Friday, 16 June 2023 at 00:35:48 UTC, Steven Schveighoffer wrote:

-Steve

How can factory functions be used to achieve effects similar to 'RAII'?
Thank you.

On Friday, 16 June 2023 at 01:00:05 UTC, Steven Schveighoffer wrote:

Thank you for your tip.
If could simplify it a bit more, it would be even better. It's really uncomfortable without this().