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Showing posts from August, 2023

strcpy / strcmp implementation

 In C/C++ we have a library function called strcpy to copy the source character array to the destination character array. The C++ function details have been documented here . Today I tried to implement it in my own way. I have put forth a few conditions to implement this function, which are described below. 1. I don't want to pass the size of arrays as function parameters. I mean, the parameter gets passed implicitly 2. I don't want the program shall compile if any of the array sizes or both array sizes are zero or one. 3. There shall be no operation if source and destination arrays are the same. 4. No overflow happens if the destination array size is smaller than the source. 5. The destination array must be null-terminated after a successful copy. Below is the client code to test the implementation: int main() { // Case 1: Source and destination arrays are of the same size char src[] = "Hello World"; char dest[12] = {}; // n - 1 chars will be copied and the las

Accessing private data member of a class without friend

R ecently dug into the object layout and observed a loophole in C++. We know in C++ we have something called reinterpret_cast. If I need to hack the access specifier of a class not defined by us may try to use reinterpret_cast to hack it, though it is illegal but it works.  Let's see the code:  I have a class defined in a file called Demo.hpp. The class definition looks like below: class Demo { private:     int x;     char ch; public:     void print()     {         std::cout << "The value of x is: " << x << "\n";     } }; Now I have created a class of similar structure but not exactly the same but somewhat similar to the Demo class.  class hack { public:     int a; }; int main() {     Demo d;     // Through reinterpret_cast     (reinterpret_cast<hack&>(d)).a = 20;     d.print(); } To my surprise, I am able to set the value 20 to the private variable x of the class Demo. Looks like the compiler is trying to align the object model in th

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

Recover from exception and re-execute the code.

 In this post, I am trying to show how can we catch an exception, then correct the cause of the exception and re-execute the code. In Windows, we have something called SEH (Structured Exception Handling) using which we can have a chance to recover from exceptions and re-execute the code which originally raised an exception. So what I am trying to do here in my sample code is: 1. Accepting two numbers from the user and deliberately taking the denominator as 0; 2. Trying to divide by numerator with zero. 3. Division with zero results as an exception. 4. Catching the exception and asking the user to re-enter the denominator with a non-zero value and then executing the division part of the code again.  Simple! This same thing can be done for other type of exceptions too. Let's see how it works in the below demo code! #include <windows.h> #include <iostream> class ADemoToRecover { public:         int numerator = 0;         int denominator = 0; public:     int recoverFromDivi