Write HTML using C++ templates. The HTML document is represented as a single,
deeply-nested type which is type-checked by the compiler using certain rules about how
HTML elements are allowed to be nested (e.g. nothing can be a child of a <br> tag).
If compilation succeeds, you will have a program that prints a properly-indented HTML document to the standard output when run.
Say you want to write the following HTML page:
<html>
<head>
<title>Help Me.</title>
</head>
<body>
<h1>The horror!</h1>
<p>
Someone has probably done this before, but I can see why it didn't catch on.
</p>
<a href="https://github.com/csb6/html-plus-plus">For science</a>
</body>
</html>Here is a C++ program that can be used to generate that page:
#include <iostream>
#include "html++.h"
int main()
{
html<
head<
title<"Help Me.">
>,
body<
h1<"The horror!">,
p<"Someone has probably done this before, but I can see why it didn't catch on.">,
a<"href=https://github.com/csb6/html-plus-plus", "For science">
>
> page;
std::cout << page.content;
return 0;
}This library requires C++20. It works with GCC 9.2.0 with -std=c++2a enabled.
It doesn't work with Apple Clang 11.0, but it might work on other compilers. If
C++20 is fully implemented on a given compiler, it should be able to compile.
Simply #include html++.h. It is the only file you need.
I was writing some HTML, and I realized that the structure and syntax of HTML tags was quite a bit like the structure/syntax of C++ templates. Both enable you to nest identifiers into a tree structure.
Since variadic templates were added to C++, a template can hold any number of other types in a parameter pack, enabling parent nodes to hold any number of child nodes. This is necessary in order for HTML elements to be properly represented by C++ types.
Since C++20, it is now possible to use string literals as non-type template
parameters (e.g. h1<"This is a title">), making C++ templates capable of
imitating the appearance of HTML tags even more closely.
I thought I'd see how horrible it would be, and, as expected, it is pretty ridiculous.
The entire library is basically a fancy way of concatenating strings.
Each tag is defined as its own template struct
(e.g. template<...> struct h1 { ... };). Each tag takes 0 or more type/
non-type template parameters. Template parameters can be HTML attributes
(e.g. "img<src='pic.png'", "alt='A picture'">) or an arbitrarily long list of
other element types, which can themselves hold other types as child nodes
(e.g. html<head<title<...>>, body<...>>).
Type safety can be achieved by defining only template parameters that make sense for
a tag (e.g. <img> is a self-closing tag, so it would not make sense for it to accept a
template parameter pack of child nodes). Using inheritance and
static_assert, along with "phantom" types (e.g. img inherits from an empty struct
named body_element_tag), ensures that the tags make semantic sense as children
of a given node. In this way, HTML can be given a degree of type-checking.
The output text is assembled by pre-order traversing the tree of types, calling each type's
constructor recursively. Each element adds its opening tag (e.g. <html>) to a string
that is then passed by reference to each child element recursively. Once all children have
added their opening tags, each node adds its closing tag (e.g. </html>) and returns from its
constructor. The string is stored in a member of the top-level node (html) and can be
printed and/or used like a normal string at runtime. The string is assembled at runtime;
however, the structure of the document is defined at compile-time.
Probably not. However, I think the type-checking aspect could be useful. I haven't added all HTML tags, but in theory this library could be extended in such a way that you could write HTML with somewhat strong typing, which might be useful for ensuring HTML standards conformance.
Hope this project is interesting (and concerning) to you!
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