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Add RealWorld example of Builder
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‎src/Builder/RealWorld/Output.txt‎

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<body>
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<h1>Title of the Page</h1>
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<h2>Subtitle A</h2>
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<p>Lorem ipsum dolor sit amet, ...</p>
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<h2>Subtitle B</h2>
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<p>... consectetur adipiscing elit.</p>
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</body>

‎src/Builder/RealWorld/main.cc‎

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#include <iostream>
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#include <string>
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#include <vector>
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/**
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* EN: Real World Example for the Builder Design Pattern (C++03/11 Evolution)
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*
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* Need: Consider a representation of the Document Object Model in which
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* each HTML element is a non-trivial graph (multi-way tree) structure
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* whose construction is complicated by the need to add an arbitrary number of
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* children to the root.
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*
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* Solution: A HTML ElementBuilder can be used for stepwise construction of an
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* Element using an implementational variant of the Builder Design Pattern
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* known as the \e Fluent Builder. Although modern C++17/20/23 provides the
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* neccesary built-in language mechanics (i.e. initializer_list and parameter
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* packs) for a Builder, this specific \e Fluent Builder is a class that can be
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* applied to legacy code relying on the older C++03/11 standards.
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*/
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#include <iostream>
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#include <string>
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#include <vector>
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/**
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* EN: These preprocessor directives allow this standalone code to target both
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* pre- and post-C++11 standards when it comes to std::vector, in particular,
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* appending a new element as well as iterating over all of the elements. In
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* addition, unscoped C++03 and scoped C++11 enums are also handled using the
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* same technique. However, this approach is for only demonstration purposes in
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* order to show the subtle difference in the C++03- and C++11-subvariants of
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* the Fluent Builder as part of the evolution of the design pattern itself.
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*/
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#if (defined(_MSVC_LANG) && _MSVC_LANG >= 201103L) || \
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((!defined(_MSVC_LANG)) && __cplusplus >= 201103L)
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#define append_element(tag, content) emplace_back(tag, content)
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#define ranged_for(children) for (auto const &child : children)
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#define ENUMMERATION_TYPE() enum class
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#define TAG_SCOPE() html::Tag
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#else
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#define append_element(tag, content) push_back(Element(tag, content))
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#define ranged_for(children) \
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for (std::vector<Element>::const_iterator it = children.begin(); \
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it != children.end(); ++it)
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#define child *it
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#define ENUMMERATION_TYPE() enum
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#define TAG_SCOPE() html
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#endif
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/**
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* EN: The html namespace contains the core machinery of the Fluent Builder
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* Pattern, namely, the Element and ElementBuilder classes. To showcase the
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* versatility of the pattern in being able to extend the Element class with
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* different types of HTML elements (tags), a print method that relies on
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* various tags is provided to show the Fluent Builder in action.
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*/
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namespace html {
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/**
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* EN: The forward declaration for the ElementBuilder is necessary as it is
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* a friend class of the Element class in this Fluent Builder implementation.
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*/
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class ElementBuilder;
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/**
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* EN: Enumeration to represent different HTML elements. (Note that in C++11
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* the enumeration will be class-scoped.) There is also a naive helper function
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* to convert the names into strings, which is used inside of the print method.
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*/
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ENUMMERATION_TYPE() Tag{body, h1, h2, p, /* ... */};
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std::string to_string(Tag tag) {
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switch (tag) {
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case TAG_SCOPE()::body:
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return "body";
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case TAG_SCOPE()::h1:
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return "h1";
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case TAG_SCOPE()::h2:
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return "h2";
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case TAG_SCOPE()::p:
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return "p";
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/* ... */
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default:
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return "tag";
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}
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}
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/**
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* EN: This client-facing Element class is essentially a tree node that
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* stores its children by value in a dynamic container. The Fluent Builder
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* provides a means to construct an instance of root Element node and then add
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* an arbitrary number of children Element nodes.
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*/
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class Element {
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public:
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Element(Tag tag, std::string const &content = std::string())
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: tag_(tag), content_(content) {}
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/**
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* EN: The print method generates markup. Note that the ranged-based for
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* loop over the children differs between the respective C++03 and C++11
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* standards.
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*/
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friend std::ostream &operator<<(std::ostream &os, Element const &e) {
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os << "<" << to_string(e.tag_) << ">";
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if (!e.content_.empty()) {
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os << e.content_;
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} else {
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os << "\n";
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}
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ranged_for(e.children_) { os << child; }
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os << "</" << to_string(e.tag_) << ">\n";
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return os;
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}
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private:
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friend class ElementBuilder;
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private:
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Tag tag_;
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std::string content_;
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std::vector<Element> children_;
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};
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/**
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* EN: The Fluent Builder is named for its method chaining as the modifier
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* (setter) method add_child() returns the builder itself, and so it can be
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* repeatedly called to construct a complex Element with many Element children.
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*
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* Again note that that element addition operation on the vector of children
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* differs between the C++03 and C++11 standards; in the former case, the
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* Element constructor must be called explicitly whereas in the latter case, the
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* arguments are forwarded to the Element constructor.
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*/
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class ElementBuilder {
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public:
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explicit ElementBuilder(Tag tag, std::string const &content = std::string())
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: root_(Element(tag, content)) {}
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ElementBuilder &add_child(Tag tag,
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std::string const &content = std::string()) {
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root_.children_.append_element(tag, content);
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return *this;
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}
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operator Element() const { return root_; }
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private:
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Element root_;
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};
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} // namespace html
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int main() {
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html::Element body =
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html::ElementBuilder(TAG_SCOPE()::body)
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.add_child(TAG_SCOPE()::h1, "Title of the Page")
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.add_child(TAG_SCOPE()::h2, "Subtitle A")
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.add_child(TAG_SCOPE()::p, "Lorem ipsum dolor sit amet, ...")
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.add_child(TAG_SCOPE()::h2, "Subtitle B")
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.add_child(TAG_SCOPE()::p, "... consectetur adipiscing elit.")
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/* ... */;
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std::cout << body;
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}

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