1proj.html

来自「C ++ in action」· HTML 代码 · 共 34 行

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	<title>Software Project</title>
    <meta  name="description" content="Design of a calculator in C++">
    <meta name="keywords" content="top-down, design, calculator, scanner, parser, symbol table, function pointer, switch">
	<link rel="stylesheet" href="../../rs.css">
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<h2>Software Project</h2>

<p class=topics>Top down design and implementation. Scanner, top-down recursive parser, symbol table. The grammar, problems with associativity. Allocation and ownership of objects returned from a function. Function pointers, switch statements.

<p>Programmers are rarely totally satisfied with their code. So after I抳e written the program for this chapter梐 simple parser-calculator桰 immediately noticed that it could have been done better. Especially the top-level view of the program was not very clear. The nodes of the parsing tree needed access to such components as the <var>Store</var> (the calculator抯 &quot;memory&quot;), the symbol table (to print meaningful error messages), maybe even to the function table (to evaluate built-in functions). I could have either made all these objects global and thus known to everybody, or pass references to them to all the nodes of the tree. 
<p>The order of construction of these objects was important, so I felt that maybe they should be combined into one high-level object, the <var>Calculator</var>. The temptation to redesign and rewrite the whole program was strong. Maybe after the nth iteration I could come up with something close to the ideal? 
<p>Then I decided I shouldn抰 do it. Part of making a program is coming up with not-so-good solutions and then improving upon them. I would have cheated if I had come up with the best, and then reverse engineered it to fit the top-down design and implementation process. So here it is梩he actual, real-life process of <a href="2spec.html">creating a program</a>.


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