c++ - 使用不同的符号表重新计算 Boost Spirit 解析的属性的最有效方法是什么？

Question

我正在使用 Boost Spirit 在某些软件中实现功能，允许用户输入将重复应用于输入流的数学方程式。输入流值使用 boost::spirit::qi::symbols 表示为符号，用户可以在他们的等式中引用这些符号。 (例如 out1 = 3 * in1 + in2)

解析和编译用户的方程式对性能不敏感，但计算其输出值是因为它构成了时间关键管道的一部分。

在文档中使用 Spirit 的标准方式是在输入被解析时计算输出(属性)。然而，由于在每次计算之间只有符号的属性值(out1、in1 等)会发生变化，所以感觉可能有更有效的方法来实现这一点，或许可以通过缓存表达式的抽象语法树并重复它来实现。

在给定一组新的符号值的情况下，重新计算此(固定)等式值的最有效方法是什么？

Answer 1

Spirit 的标准使用方式并不像您想象的那么有限。

虽然您可以使用它来计算即时值，但更常见的是在输出属性中构建一个 AST 树，它可以被转换(简化、优化)和解释(例如发出虚拟机甚至汇编指令)。

编译器教程完整地展示了这一点，但计算器示例非常接近您似乎正在寻找的内容:http://www.boost.org/doc/libs/1_55_0/libs/spirit/example/qi/compiler_tutorial/

• calc1 in example/qi/compiler_tutorial/calc1.cpp

  Plain calculator example demonstrating the grammar. The parser is a syntax checker only and does not do any semantic evaluation.

• calc2 in example/qi/compiler_tutorial/calc2.cpp

  A Calculator example demonstrating the grammar and semantic actions using plain functions. The parser prints code suitable for a stack based virtual machine.

• calc3 in example/qi/compiler_tutorial/calc3.cpp

  A calculator example demonstrating the grammar and semantic actions using phoenix to do the actual expression evaluation. The parser is essentially an "interpreter" that evaluates expressions on the fly.

这是您感兴趣的地方，因为它在解析期间停止进行计算:

• calc4 in example/qi/compiler_tutorial/calc4.cpp

  A Calculator example demonstrating generation of AST. The AST, once created, is traversed,

  1. To print its contents and
  2. To evaluate the result.
     
     

• calc5 in example/qi/compiler_tutorial/calc5.cpp

  Same as Calc4, this time, we'll incorporate debugging support, plus error handling and reporting.

• calc6 in example/qi/compiler_tutorial/calc6.cpp

  Yet another calculator example! This time, we will compile to a simple virtual machine. This is actually one of the very first Spirit example circa 2000. Now, it's ported to Spirit2.

• calc7 in example/qi/compiler_tutorial/calc7/main.cpp

  Now we'll introduce variables and assignment. This time, we'll also be renaming some of the rules -- a strategy for a grander scheme to come ;-)

  This version also shows off grammar modularization. Here you will see how expressions and statements are built as modular grammars.

• calc8 in example/qi/compiler_tutorial/calc8/main.cpp

  Now we'll introduce boolean expressions and control structures. Is it obvious now what we are up to? ;-)

我相信您会在本教程结束时找到很多灵感!