[boost] Purpose adding a scope guard classes to the Boost library

21 Aug 2005

      ...
, // Exceptions to be caught (up to 10 exception types supported)
            boost::shared_ptr< CHandler > // An optional parameter describes
Sometimes it is very useful to register some postponed actions regardless of 
some part of the program result (a function, for example). Smart pointers 
are obvious example of such actions (namely, freeing resources) and in fact 
are a special case of so called scope guards. Guards may be especially 
useful in heavilly-logical code with multiple (maybe folded into each other) 
checks or in code using exceptions.

As a simple example, suppose we have a logging system that may trace 
function calls in our program. The easiest and obvious way to implement such 
functionality is to create some object in the beginning of each function to 
be traced. This object will add the function to the logger on its 
construction and remove it on destruction.
Another application is resolving circular calling dependency issues which 
tend to come up in rather complex projects. The following example 
illustrates the problem:

class A;

// The objects of class B are managed by object of class A
class B {
public:
    void execute(A* pA, int n);
};

// The object of main class A manages some other objects of class B
class A {
    // Some internal data, which is provided to the managed objects
    std::set< int > m_Data;

public:
    // The entrance method, called from somewhere outside
    void process(B* pB) {
        // Imagine we have some real processing here and m_Data is not empty
        pB->execute(this, *m_Data.begin());
        // Here we cannot be absolutely sure if m_Data still contains the 
element we passed in line above
    }

    void remove(int n) {
        m_Data.erase(n);
    }
};

void B::execute(A* pA, int n) {
    if (n > 10)
        pA->remove(n);
}

The case above is usually fixed by using some kind of state variables or 
flags or, in more complex cases, some stacks of postprocessing. I.e. in 
A::remove we don't actually erase the element but schedule some action to 
perform when control returns to A::process. This can easily be done by 
enqueued functors or scope guards. In some cases the latter are simplier to 
use and more laconic.

I purpose to add a generalized scope guard classes to the Boost library. I 
have a rather simple but flexible implementation of such classes (I've 
attached the sources). Since I'm new here, I don't really know if they 
conform some source code conventions in Boost (if any), but if they don't - 
please, let me know.

Few general words about the implementation. There are two general guard 
classes: leaving_scope_guard and scope_guard. The first one calls a functor 
(boost::function is used) with no arguments on its destruction if it is 
armed. The second class extends the first one by calling another functor on 
its construction (this is not a functionality of real necessity, but it 
allows to semantically group paired functions invocation, see the example 
above). A guard once created may be armed or disarmed later in thÕ program, 
but the armed status applies only to the destructing functor invoking (the 
constructing one is controlled via initial status of the guard which is 
passed in guard constructor arguments).
The flexibility of implementation is in ability to change the functors 
invoking policy. In the simpliest case it is a direct invoking (all template 
parameters by default), but there is a policy with exception catching 
support. See the example:

struct my_exception : public std::exception { /* ... */ };

void guarded_foo(int n, const char* p); // throws bad_cast, bad_alloc and 
my_exception

// This is the class of the exception handler object
struct CHandler {
    void on_exception(std::bad_cast& e) throw() {
        std::cout << "on_exception called, bad_cast caught" << std::endl;
    }
    void on_exception(std::bad_alloc& e) throw() {
        std::cout << "on_exception called, bad_alloc caught" << std::endl;
    }
    void on_exception(my_exception& e) throw() {
        std::cout << "on_exception called, my_exception caught" << 
std::endl;
    }
};

void foo(int n) {
    leaving_scope_guard<
        guard::with_catcher< // Indicate that we're expecting some 
exceptions to be thrown by guarded_foo
            CHandler, // This is the class, which object is to process 
exceptions caught
            guard::exceptions< std::bad_cast, std::bad_alloc, my_exception 
the way the pointer to CHandler object should be stored. The default value 
would be CHandler*.
        >
    > guard(boost::bind(&guarded_foo, n, "called from foo"), 
boost::shared_ptr< CHandler >(new CHandler)); // Guard object

    // ...
}

There also are macroses defined for setting anonymous scope guards when 
there is no need for arming or disarming it.

As for compatibility, I successfully compiled it on VC 7.1, ICC 9.0 and GCC 
3.4.4 (MinGW version). I don 't think it will compile on VC 6 since explicit 
template function instantiation is used there which is not supported by the 
compiler, but I think it is possible to work around it. Besides that I don't 
think there should be any problems for other compilers.

Any suggestions are welcome. 

begin 666 scope_guard.h
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`
end

[boost] Purpose adding a scope guard classes to the Boost library

Andrey Semashev