Question

I am trying to implement something similar to the Python with statement in C++. As I plan to use it mainly with Qt-OpenGL the methods are called bind and release (in Python __enter__, __exit__).

Code I came up with:

header:

#include <iostream>
#include <vector>

class With
{
public:
    class A
    {
    public:
        virtual ~A() { }
    };

    template <typename T>
    class B : public A
    {
    public:
        B(T& _t) : t(_t)
        {
            t.bind();
        }

        virtual ~B()
        {
            t.release();
        }

        T& t;
    };

    template <typename... Args>
    With(Args&... args)
    {
        set(args...);
    }

    ~With();

    template <typename T, typename... Args>
    void set(T& t, Args&... args)
    {
        set(t);
        set(args...);
    }

    template <typename T>
    void set(T& t)
    {
        a.push_back(dynamic_cast<A*>(new B<T>(t)));
    }

    std::vector<A*> a;
};

cpp:

With::~With()
{
    for (auto it = a.begin(); it != a.end(); ++it)
    {
        delete *it;
    }
}

Usage:

class X
{
public:
    void bind() { std::cout << "bind x" << std::endl; }
    void release() { std::cout << "release x" << std::endl; }
};

class Y
{
public:
    void bind() { std::cout << "bind y" << std::endl; }
    void release() { std::cout << "release y" << std::endl; }
};

int main()
{
    X y;
    Y y;

    std::cout << "start" << std::endl;
    {
        With w(x, y);
        std::cout << "with" << std::endl;
    }

    std::cout << "done" << std::endl;

    return 0;
}

Questions:

1. Needing class A and class B feels a bit clumsy. Is there a better alternative?
2. Are there any draw backs in using && instead of &? It would make the usage of tempory objects possible (e.g. With w(X(), y);)

Answer 1

The with statement is a way to do in python what is already the normal thing in C++. It is called RAII: Resource acquisition is initialization.

In python, when a class object is created, the __init__ method is called (but this is not a strict guarantee). The __del__ method is called by the garbage collector at some point after the object is no longer in use, but it is not deterministic.

In C++ the destructor is called at a well defined point so there is no need for with.

I suggest you just use something like class B (no need for class A or With).

template <typename T>
class B {
public:
    B(T& t) : m_t(t){
        m_t.bind();
    }
    ~B() {
        m_t.release();
    }
    T& m_t;
}

use it like this:

{
    B<X> bound_x(x);  // x.bind is called
    B<Y> bound_y(y);  // y.bind is called
    // use x and y here
} // bound_x and bound_y is destroyed here 
  // so x.release and y.release is called