Relevancy: 1.9 stable
In Rust there is no concept of a constructor as a language feature, like for example in C++. However there is a strong convention (and mentioned in the guidelines) to use a static method called new as the constructor. This works well, but you can have only one function called new in the impl - there is no method overloading. So are we out of luck if we want to have different constructors? No! But arguably different purposes should imply different method names, so another convention is to prefix extra constructors with with_ (such as Vec::with_capacity) or from_, if the constructor does some kind of a conversion.
A few types in the standard library and some third-party crates provide a from_fn constructor. This curious method usually takes as arguments some sort of dimensions and a closure that will generate values.
In the nalgebra crate there is a DMat type representing a matrix which dimensions are known at runtime. We can build a matrix using the from_fn constructor too. Let's create a triangular matrix:
include:2-2 include:6-6 include:17-18
The first two arguments to from_fn are numbers of rows and columns; this means the closure must take two arguments - indices of the current row and column. And here's our matrix:
$ cargo run
1 0 0 0 0 0 0
1 1 0 0 0 0 0
1 1 1 0 0 0 0
1 1 1 1 0 0 0
1 1 1 1 1 0 0
1 1 1 1 1 1 0
1 1 1 1 1 1 1Bur we're not limited to mathematical objects. For example the image crate provides a buffer that can generate the image with the from_fn call.
include:1-1 include:5-5 include:19-24
We're working in two dimensions in the same manner as with the DMat type. And here's the generated image: