### swap using XOR

In one of my previous posts I described a few ways of swapping two numbers using xor operation in a sinlge line. The discussion is not correct or rather incomplete in the sense that the algorithm needs to be guarded by an if condition. The algorithm that exercises proper care of sequence points is as below:

swap (int &a, int &b) {
if (&a != &b) {
a ^= b;
b ^= a;
a ^= b;
} }

It is crutial that the two number must not be located at the same address. The two numbers must have different identities. The algorithm works even though the values are the same.

### Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together.

It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array?

int native[3][4];
std::array<std::array<int, 3>, 4> arr;

No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is:

std::array<std::array<int, 4>, 3> arr;

That's quite annoying for two r…

### Inheritance vs std::variant

C++17 added std::variant and std::visit in its repertoire. They are worth a close examination. I've been wondering about whether they are always better than inheritance for modeling sum-types (fancy name for discriminated unions) and if not, under what circumstances they are not. We'll compare the two approaches in this blog post. So here it goes.

Inheritancestd::variantNeed not know all the derived types upfront (open-world assumption)Must know all the cases upfront (closed-world assumption)Dynamic Allocation (usually)No dynamic allocationIntrusive (must inherit from the base class)Non-intrusive (third-party classes can participate)Reference semantics (think how you copy a vector of pointers to base class?)Value semantics (copying is trivial)Algorithm scattered into classesAlgorithm in one placeLanguage supported (Clear errors if pure-virtual is not implemented)Library supported (poor error messages)Creates a first-class abstractionIt’s just a containerKeeps fluent interfaces…

### Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example.
struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a c…