Skip to main content

Forgotton friend: pointer/reference to an array

The name of an array "degenerates" into a pointer to the first element of the array. For example,
int Array [10];
int *p = Array;

there is a quite a bit loss of information here. Specifically, such a decay loses its type information and specifically, its dimensions. The type of Array is "An integer array of 10 integers." So we lost the word "array" and we also lost the length (10) of the array.

An advantage of using pointer to an array is that this type information is retained. int (*q)[10] = &Array; It declares a pointer q to to an array of 10 integers. So whats the big deal? Compiler has this information and it is happy to let us take advantage of it.

template <int p, int q, int r>
int average (int (&array)[p][q][r]) { ... }

main () {
int q [][2][2] = {
{ {1, 2}, {3, 4} },
{ {5, 6}, {7, 8} }
};
average (a); /// This call knows the dimensions of the array.
}
This type information can be exploited to implement the average function using template meta-programming techniques! This can be useful in generative programming as well.

Comments

Anonymous said…
hi sumant,
place your google adsense correctly across the page for better results.

kaushal chandak
Forgotten friend :)
Sumant said…
Thanks Kaushal!
I have put some google ads just for kicks! I don't want the ads to cause difficulty while reading. C++ Truths is for non-profit!

Popular Content

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 annoyin

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

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in