Implementing C++ equivalent of C# using statement

Question

I am looking for an elegant solution for implementing the equivalent of the C# using statement in C++. Ideally the resultant syntax should be simple to use and read.

C# Using statement details are here - http://msdn.microsoft.com/en-us/library/yh598w02(v=vs.80).aspx

I am not sure whether the solution would be to use function pointers with destructors on classes, some form of clever template programming or even meta template programming. Basically I do not know where to start with this...

Answer 1

You don't need to implement this in C++ because the standard pattern of RAII already does what you need.

{
    ofstream myfile;
    myfile.open("hello.txt");
    myfile << "Hello\n";
}

When the block scope ends, myfile is destroyed which closes the file and frees any resources associated with the object.

The reason the using statement exists in C# is to provide some syntactic sugar around try/finally and IDisposable. It is simply not needed in C++ because the two languages differ and the problem is solved differently in each language.

Answer 2

First, we have to define a Closeable/Disposable public interface:

#include <iostream>

using namespace std;


class Disposable{
private:
    int disposed=0;
public:
    int notDisposed(){
        return !disposed;
    }
    
    void doDispose(){
        disposed = true;
        dispose();
    }
    
    virtual void dispose(){}
    
};

Then we should define a macro for the using keyword:

#define using(obj) for(Disposable *__tmpPtr=obj;__tmpPtr->notDisposed();__tmpPtr->doDispose())

and; here is an example application:

class Connection : public Disposable {
    
private:
    Connection *previous=nullptr;
public:
    static Connection *instance;
    
    Connection(){
        previous=instance;
        instance=this;
    }
    
    void dispose(){
        delete instance;
        instance = previous;
    }
};

Connection *Connection::instance = nullptr;

int Execute(const char* query){
    if(Connection::instance == nullptr){
        cout << "------- No Connection -------" << endl;
        cout << query << endl;
        cout << "------------------------------" << endl;
        cout << endl;
        
        return -1;//throw some Exception
    }
    
    cout << "------ Execution Result ------" << endl;
    cout << query << endl;
    cout << "------------------------------" << endl;
    cout << endl;
    
    return 0;
}

int main(int argc, const char * argv[]) {
    
    using(new Connection())
    {
        Execute("SELECT King FROM goats");//in the scope 
    }
    
    Execute("SELECT * FROM goats");//out of the scope
    
}

But if you want to delete variables automatically from memory, you can simply use braces {}; therefore, every variable inside of the scope will be removed at the end of the scope. here is an example:

int main(int argc, const char * argv[]) {
    {
        int i=23;
    } 
    
    // the variable i has been deleted from the momery at here.
}