Execute some code when an object goes out of scope

Question

There are times where I want to be sure that some function call is executed before returning from an other function, or an integer is decremented.

For example consider this code:

void doSomething()
{
    try
    {
        .....
        .....
        return;
        .....
        .....
        .....
        return;
        ......
    } catch ( ... )
    }
    myvariable--;
}

I want to be sure that myvariable is always decremented, no mater if there is an exception, or a return call. Is there any build in object in C++ for this, or I should create my own class that accepts an std::function and executes it when the object gets out of scope at the object destructor?

For example, in C# there is the try..catch...finally block where in finally you can put any code you want to execute before returning from the function.

Answer 1

You can write your own, class, that performs some action, in its destructor.

class DoAction
    {
public:
    DoAction (std::function<void ()> func) : m_func (func)
        {
        }
    ~DoAction ()
        {
        m_func ();
        }

private:
    std::function<void ()> m_func;
    };

And then, create an object of it, in a following way:

DoAction action([&] {/*Code that you want to be executed upon leaving the scope.*/});

Live demo.

Answer 2

I often use std::unique_ptr for this, using lambda as custom deleter. It's slightly awkward, since you have to do decltype() for deleter type, but other than that, it saves you writing a 10-liner class.

Answer 3

You can do something like this:

#include <iostream>
template <typename T>
struct decrementer_raii
{
   T& var;
   decrementer_raii(T& v) : var(v) {}
   ~decrementer_raii() { --var; }
};

int main()
{
    int x = 1;
    {
       decrementer_raii<int> dec(x);
    }
    std::cout << x;  // should print 0.
}

Live Example

or more like your scenario:

#include <iostream>
template <typename T>
struct decrementer_raii
{
   T& var;
   decrementer_raii(T& v) : var(v) {}
   ~decrementer_raii() { --var; }
};

void foo(int& x)
{
   decrementer_raii<int> dec(x);
   std::cout << x << "\n"; // x is still the original value passed-in (14)
   for (int i = 0; i < 10; ++i )
   {
      if ( i == 4 ) throw 10;  // force a throw 
   }
}

int main()
{
    int x = 14;
    try {
        foo(x);
    }
    catch(...) {}
    std::cout << x;  // should print 13.
}

Live Example 2