How can C 11 Lambda Expressions be Used to Implement Scope Guards?

Scope Guards in C 11

One of the main advantages of C 11 lambda expressions lies in their ability to express the concept of RAII (Resource Acquisition Is Initialization) in a concise and elegant manner. The traditional implementation of RAII relies on destructors to release resources, but with lambda expressions, it is possible to create an object that will execute a cleanup function upon exiting its scope, regardless of how that exit occurs. This is known as a scope guard.

Simple Scope Guard Implementation

Here is an example of a simple scope guard implementation in C 11:

<code class="c++">template< typename Lambda >
class ScopeGuard
{
    mutable bool committed;
    Lambda rollbackLambda;
    public:

        ScopeGuard( const Lambda&amp; _l) : committed(false) , rollbackLambda(_l) {}

        template< typename AdquireLambda >
        ScopeGuard( const AdquireLambda&amp; _al , const Lambda&amp; _l) : committed(false) , rollbackLambda(_l)
        {
            _al();
        }

        ~ScopeGuard()
        {
            if (!committed)
                rollbackLambda();
        }
        inline void commit() const { committed = true; }
};</code>

The ScopeGuard class takes a lambda expression as its constructor argument, which represents the cleanup action to be executed when the scope guard goes out of scope. The committed flag indicates whether the cleanup action has been executed.

Usage

The ScopeGuard class can be used to ensure that resources are released properly, even if an exception occurs. For example, the following function acquires a resource and ensures that it is released using a scope guard:

<code class="c++">void SomeFuncThatShouldBehaveAtomicallyInCaseOfExceptions() 
{
    std::vector<int> myVec;
    std::vector<int> someOtherVec;

    myVec.push_back(5);
    //first constructor, adquire happens elsewhere
    const auto&amp; a = RAII::makeScopeGuard( [&amp;]() { myVec.pop_back(); } );  

    //sintactically neater, since everything happens in a single line
    const auto&amp; b = RAII::makeScopeGuard( [&amp;]() { someOtherVec.push_back(42); }
                     , [&amp;]() { someOtherVec.pop_back(); } ); 

    b.commit();
    a.commit();
}</code>

In this example, the ScopeGuard is used to ensure that the elements are removed from the vectors even if an exception occurs.

Conclusion

Scope guards are a powerful tool for ensuring that resources are released properly in C 11. The simple implementation presented here is a straightforward way to implement this pattern. However, there are more complex implementations that offer additional features, such as the ability to transfer ownership of resources between scope guards.

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