Computation on structure

Visitor pattern非常适合用于computation on structure，visitor所对应的是一个function pointer、computation，下面是典型的例子。

Examples

In compiler

wikipedia Abstract syntax tree # Design patterns

在wikipedia Abstract syntax tree的Design patterns章节，总结了 visitor pattern 在compiler的实现中的应用：

Because the compiler traverses the tree several times to determine syntactic correctness, it is important to make traversing the tree a simple operation. The compiler executes a specific set of operations, depending on the type of each node, upon reaching it, so it often makes sense to use the visitor pattern.

这段话中的“the tree”只的是compiler构造的abstract syntax tree。compiler需要多次遍历这棵树，每次遍历执行一些操作，显然，这种场景是非常适合于使用visitor pattern的。

stackoverflow Visitor Pattern VS Iterator Pattern: visiting across hierarchy class?的A

在这回答中给出了:

• LLVM source code
• CPython AST

The other good example I know of shows up in abstract syntax tree manipulations. CPython and LLVM both use visitors. Using a visitor here prevents code that manipulates certain AST nodes from needing to know how to iterate over all the various AST nodes that might branch in complicated ways.

Hierarchy structure

对具有hierarchy structure的数据进行操作，都可以使用visitor pattern。

比如

1) JSqlParser

2) Mach7

3) visitor pattern的一个例子是: 在Stack Overflow中列举的：遍历一个目录结构，然后对每个file/dir都执行一个操作

The Boost Graph Library (BGL)

Extension through Visitors

Second, the graph algorithms of the BGL are extensible. The BGL introduces the notion of a visitor, which is just a function object with multiple methods. In graph algorithms, there are often several key “event points” at which it is useful to insert user-defined operations. The visitor object has a different method that is invoked at each event point. The particular event points and corresponding visitor methods depend on the particular algorithm. They often include methods like start_vertex(), discover_vertex(), examine_edge(), tree_edge(), and finish_vertex().

C++ std::visit

Visitor pattern and FP(functional programming)

在client code，可以使用FP。