Importance of 'const' in C++

Lets declare a class 'Demo'

class Demo
{
private:
 char a;

 void TestConst(char* const pData)const
 {
  // pData = "test"; // char* const protects 
  // pData++;        // pointer!

  // a = 'a';        // const for function 
                     // protects data member.
 }
};

So, when we are working with C++, all data members irrespective of access specification, can be protected with 'const', appended to the declaration/definition.

Comments

Rahul Bhattacharjee said…

do you mean data member a becomes a constant ? as you would not be able to assign any value to it. If that is the case then you have to declare and define a value of it there itself.
Is that correct ?

March 9, 2009 at 9:17 AM

Mufti Mohammed said…

This has two aspect. First look at function signature.

First, char *const pData means pData is a const pointer to char. We can't change the pointer itself.

Second TestConst is a const function. const function does not allow any modification of member variables, though member variable(s)is/are not const.

March 10, 2009 at 1:03 AM

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 ...

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 < ...

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

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