posts: add article about magic conversions in C++

content/posts/magic-conversions-in-c++.md

@@ -0,0 +1,172 @@
+---
+title: "Magic Conversions in C++"
+date: 2021-10-01T14:46:14+02:00
+draft: false # I don't care for draft mode, git has branches for that
+description: "How to get the compiler to infer the correct conversion"
+tags:
+  - c++
+  - design-pattern
+categories:
+  - programming
+series:
+favorite: false
+disable_feed: false
+---
+
+One feature that I like a lot in [Rust][rust-lang] is return type polymorphism,
+best exemplified with the following snippet of code:
+
+```rust
+use std::collections::HashSet;
+
+fn main() {
+    let vec: Vec<_> = (0..10).filter(|a| a % 2 == 0).collect();
+    let set: HashSet<_> = (0..10).filter(|a| a % 2 == 0).collect();
+    println!("vec: {:?}", vec);
+    println!("set: {:?}", set); 
+}
+```
+
+We have the same expression (`(0..10).filter(|a| a % 2 == 0).collect()`) that
+results in two totally different types of values (a `Vec` and a `HashSet`)!
+
+This is because Rust allows you to write a function which is generic in its
+*return type*, which is a super-power that C++ does not have. But is there a way
+to emulate this behaviour with some clever code?
+
+[rust-lang]: https://rust-lang.org/
+<!--more-->
+
+## The problem
+
+For the purposes of this article, the problem that I am trying to solve will be
+the following:
+
+```c++
+void takes_small_array(std::array<char, 32> arr);
+void takes_big_array(std::array<char, 4096> arr);
+
+// How to define a `to_array` function so that the following works?
+void test(std::string_view s) {
+    takes_small_array(to_array(s));
+    takes_big_array(to_array(s));
+}
+```
+
+## First attempt
+
+If we try to solve this in a way similar to Rust, we hit a problem in what the
+language allows us to write:
+
+```c++
+std::array<char, 32> to_array(std::string_view s) {
+    std::array<char, 32> ret;
+    std::copy(s.begin(), s.end(), ret.begin());
+    return ret;
+}
+
+std::array<char, 4096> to_array(std::string_view s) {
+    std::array<char, 4096> ret;
+    std::copy(s.begin(), s.end(), ret.begin());
+    return ret;
+}
+```
+
+The compiler complains with the following error:
+
+```none
+ambiguating new declaration of 'std::array<char, 4096> to_array(std::string_view)'
+note: old declaration 'std::array<char, 32> to_array(std::string_view)'
+```
+
+That is because C++ does **not** allow you to write an overload set based on
+*return type only*.
+
+## Using templates
+
+For our second try, we want to use *non-type template parameters* to solve the
+issue. We write the following:
+
+```c++
+template <size_t N>
+std::array<char, N> to_array(std::string_view s) {
+    std::array<char, N> ret;
+    std::copy(s.begin(), s.end(), ret.begin());
+    return ret;
+}
+```
+
+The compiler does not complain when we write this! We have also solved two minor
+issues with the previous try: the size of the arrays are not hard-coded, and we
+kept the code DRY.
+
+However we have some trouble trying to use those functions as stated in the
+beginning of the problem, with the following error message:
+
+```none
+error: no matching function for call to 'to_array(std::string_view&)'
+      |     takes_small_array(to_array(s));
+note: candidate: 'template<size_t N> std::array<char, N> to_array(std::string_view)'
+      | std::array<char, N> to_array(std::string_view s) {
+note:   template argument deduction/substitution failed:
+note:   couldn't deduce template parameter 'N'
+```
+
+The compiler cannot deduce the size of the array we want to use! We could solve
+the issue by explicitly giving a size when calling the function
+(`to_array<32>(s)`) however this is unsatisfactory: we are not solving the
+problem as stated initially, which could for example lead to needless churning
+if we change the signature of `takes_small_array` to instead use
+`std::array<char, 64>`).
+
+Thankfully there is a way to use the compiler to our advantage, and have it
+deduce it for us, but it involves some trickery.
+
+## The solution
+
+We want to write a function that resolves the previous two issues we
+experienced:
+
+* The non-type template parameter must be deduced by the end of the call to
+`to_array`, but we can only deduce it once it is being consumed by
+`takes_{small,big}_array` -- which is too late for the compiler.
+* We cannot overload on the return type, which means we must return a single
+type from the function.
+
+The goal is to delay *when* the deduction of the array's size is happening,
+which can be done by using a *templated conversion operator*.
+
+So the solution to our problem is to do the following:
+
+```c++
+class ToArray {
+    std::string_view s_;
+
+public:
+    ToArray(std::string_view s) : s_(s) {}
+
+    template <size_t N>
+    operator std::array<char, N>() const {
+        std::array<char, N> ret;
+        std::copy(s_.begin(), s_.end(), ret.begin());
+        return ret;
+    }
+}
+
+ToArray to_array(std::string_view s) {
+    return ToArray{s};
+}
+```
+
+The following steps happen when trying to call `takes_small_array(to_array(s))`:
+
+* `to_array(s)` returns a `ToArray` value.
+* the `ToArray` value is not an `array<char, 32>`, but has an implicit
+conversion operator, which the compiler invokes.
+* `takes_small_array` is called with the converted `array<char, 32>` value.
+
+We now have a "magic" function which can convert a `string_view` to an
+`std::array` of characters of any size. We could further improve this by
+ensuring that the array is terminated with a `'\0'`, throwing an exception when
+the array is too small for the given string, etc... This is left as an exercise
+to the reader.