What is the Managed C++ equivalent to the C# using

问题:

How would one code the following C# code in Managed C++

void Foo()
{
    using (SqlConnection con = new SqlConnection("connectionStringGoesHere"))
    {
         //do stuff
    }
}

Clarificaton: For managed objects.

回答1:

Assuming you mean C++/CLI (not the old Managed C++), the following are your options:

(1) Mimic a using-Block with using automatic / stackbased objects:

{
  SqlConnection conn(connectionString);
}

This will call the Destructor of the "conn" Object when the next enclosing block ends. Whether this is the enclosing function, or a block you manually add to limit scope doesn't matter.

(2) Explicitly call "Dispose", i.e. destruct the object:

SqlConnection^ conn = nullptr;
try
{
  conn = gcnew SqlConnection(conntectionString);

}
finally
{
  if (conn != nullptr)
    delete conn;
}

The first one would be the direct replacement for "using". The second one is an option, typically you won't need to do unless you optionally pass the reference to somewhere else.

回答2:

To to that in Managed C++ just use stack semantics.

void Foo(){
   SqlConnection con("connectionStringGoesHere");
    //do stuff
}

When con goes out of scope the "Destructor", ie Dispose(), is called.

回答3:

You could do something similar in an auto_ptr style:

void foo()
{
    using( Foo, p, gcnew Foo() )
    {
        p->x = 100;
    }
}

with the following:

template <typename T>
public ref class using_auto_ptr
{
public:
    using_auto_ptr(T ^p) : m_p(p),m_use(1) {}
    ~using_auto_ptr() { delete m_p; }
    T^ operator -> () { return m_p; }
    int m_use;
private:
    T ^ m_p;
};

#define using(CLASS,VAR,ALLOC) \
    for ( using_auto_ptr<CLASS> VAR(ALLOC); VAR.m_use; --VAR.m_use)

For reference:

public ref class Foo
{
public:
    Foo() : x(0) {}
    ~Foo()
    {
    }
    int x;
};

回答4:

#include <iostream>

using namespace std;


class Disposable{
private:
    int disposed=0;
public:
    int notDisposed(){
        return !disposed;
    }

    void doDispose(){
        disposed = true;
        dispose();
    }

    virtual void dispose(){}

};



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;


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

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");//out of the scope
    }

    Execute("SELECT * FROM goats");//in the scope

}

回答5:

If you're concerned about limiting the variable's lifetime rather than automatic disposal, you can always just put it into its own scope:

void Foo()
{
    {
        SqlConnection con = new SqlConnection("connectionStringGoesHere");
        // do stuff
        // delete it before end of scope of course!
    }
}