Skip to main content

Bind lambda with a class and execute while class instance destroys (A final pattern)

 Let's have a template class like below:

template< typename T >
class finalGuard {
    T m_func;
public:
    finalGuard(T func) : m_func(func) {}
    ~finalGuard() { m_func(); }
};

What is the requirement? I need to execute lambda when a template instance of the above class goes out of scope or any exception is caused during the execution of code. We can think of this as more or like a final pattern. When a class object's lifetime ends, a piece of code will start executing to wrap up things. 

Now I have a factory-like function defined as below to resolve type dependency which lambda is expecting:

template< typename T >
finalGuard< T > finalCall(T obj) { return obj; } 

The below piece of code is used to bind the lambda with the class instance and it gets executed as soon as a class object destructor gets invoked.

int main()
{
    try {
        auto guard = finalCall([]() noexcept {
            cout << "Good bye via exception!\n";
            });
        cout << "This is followed by a throw statement\n";
        throw runtime_error("Unknown Error"); // This is a throw which calls finalCall 
        cout << "This line will never execute\n";
    }
    catch (exception& ex) { cout << "This is caused by an " << ex.what() << "\n\n"; }

    {
        auto guard = finalCall([]() noexcept {
            cout << "Good bye via normal execution!\n";
            });
        cout << "This is a simple execution\n";
        cout << "Normal execution ends here...\n"; 
    }

    return 0;
}

So, as mentioned above code, irrespective of the execution path lambda will be executed to do some work.

The above code works with Microsoft VS 2019, and GNU C++ [C++ 11/14/17].

If we are targeting only the GNU C++ compiler then code can be stripped down a bit. In that case, we don't need a factory-like function explicitly. (Used GCC 11.1.0) 

#include <iostream>
#include <stdexcept>

using std::cout;
using std::exception;
using std::runtime_error;

template< typename T >
class finalGuard {
    T m_func;
public:
    finalGuard(T func) : m_func(func) {}
    ~finalGuard() { m_func(); }
};

/*
Resolving type dependency for lambda is not needed on GCC C++ 17 onwards.
//template< typename T >
//finalGuard< T > finalCall(T obj) { return obj; }
*/ 

int main()
{
    try {
        auto guard = finalGuard([]() noexcept {
            cout << "Good bye via exception!\n";
            });
        cout << "This is followed by a throw statement\n";
        throw runtime_error("Unknown Error"); // This is a throw which calls finalCall 
        cout << "This line will never execute\n";
    }
    catch (const exception& ex) { cout << "This is caused by an " << ex.what() << "\n\n"; }

    {
        auto guard = finalGuard([]() noexcept {
            cout << "Goodbye via normal execution!\n";
            });
        cout << "This is a simple execution\n";
        cout << "Normal execution ends here...\n"; 
    }

    return 0;
}


Labels:

Comments

Post a Comment

Popular posts from this blog

Reversing char array without splitting the array to tokens

 I was reading about strdup, a C++ function and suddenly an idea came to my mind if this can be leveraged to aid in reversing a character array without splitting the array into words and reconstructing it again by placing spaces and removing trailing spaces. Again, I wanted an array to be passed as a function argument and an array size to be passed implicitly with the array to the function. Assumed, a well-formed char array has been passed into the function. No malformed array checking is done inside the function. So, the function signature and definition are like below: Below is the call from the client code to reverse the array without splitting tokens and reconstructing it. Finally, copy the reversed array to the destination.  For GNU C++, we should use strdup instead _strdup . On run, we get the following output: Demo code
Post a Comment
Read more

XOR (Exclusive OR) for branchless coding

The following example shows the array reversing using the  XOR operator . No need to take any additional variable to reverse the array.   int main(int argc, _TCHAR* argv[]) { char str[] = "I AM STUDENT"; int length = strlen(str); for(int i = 0; i < ((length/2)); i++) { str[i] ^= str[length - (1+i)]; str[length - (1+i)] ^= str[i]; str[i] ^= str[length - (1+i)]; } cout << str << endl; return 0; } The above example is one of the uses of XOR but XOR comes in handy when we can do branchless coding  methods like butterfly switch etc. Sometimes this is very effective in speeding up the execution.  Let's see one of the uses of XOR in branchless coding. I am taking a simple example of Y = | X |.  Yes, I am generating abs of a supplied number. So, my function signature/definition in C++ looks like below: int absoluteBranch( int x) {     if (x < 0 ) {         return -x;     }     else {         retur
1 comment
Read more

Power of Two

  I n this post will be discussing how to calculate if a number is a power of two or not. As an example, 8 is a power of two but the number 10 is not. There are many ways we can solve this. First , we will take an approach which is simple and iterative. In this case, we will calculate the power of two one by one and check with the supplied number. The below code illustrates it. bool isPowerofTwo(unsigned num) { auto y = 1; while (0 != y) { if (num == y) return true; if (num < y) return false; y <<= 1; } return false; } Second , assuming, the number is a 32-bit number, this is also an iterative solution. In this scenario, iterating all bits and counting the set bits. Any number which is a power of 2 will have only one bit set and the rest will be zeros. As an example, 8 in binary representation is 1000. Using this observation, we can implement an iterative solution. bool isPowerofTwo(unsigned num) { auto one_count = 0; for (auto index = 0; index < 32;
Post a Comment
Read more