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Effective STL 笔记 -- Item 9: Choose carefully among erasing options

Effective STL 笔记: Item 6--Be alert for C++'s most vexing parse

tubo posted @ 2014年9月03日 00:10 in 未分类 , 736 阅读

 

假设有个文件里面记录的一系列的 int 值，现在我们想把这些数值存到一个 List 里面，结合 Item 5, 我们可能会写出下面的代码：

ifstream dataFile("ints.data");
list<int> data(istream_iterator<int>(dataFile), // Start of iterator
               istream_iterator());             // End of iterator

这段代码可以编译，但运行时并不工作，它不会去调用 list 的构造函数，从而不会生成我们想要的这个 List。

问题，出在 C++ 对代码的解析上。

假设我们需要声明一个函数，该函数接受 double 类型参数并返回 int 类型，C++ 里面，下面三种方法是等效的：

1: int f(double d);  // Old C style.
2: int f(double(d)); // Function style casts.
3: int f(double);    // Same as first but skip parameter.

如果我们要声明另外一个函数，该函数同样返回 int，但接受的参数是一个无参数但返回 double 的函数的指针，则下面的声明是等效的：

4: int g(double (*pf)()); // g takes a pinter to a function as paramter.
5: int g(double pf());    // same as above
6: int g(double ());      // same as above, but parameter (function name) skipped.

观察 1 ~ 6 我们可以看到“ 括号”在不同位置时候的不同作用：

  • 参数 周围的括号可以被忽略
  • 单独 的括号实际上意味着这是一个函数指针的参数列表！

了解了这个区别之后再返回来看最开始的那个声明:

list<int> data(istream_iterator<int>(dataFile), istream_iterator<int>());

这个声明定义了一个返回 list<int> 的函数，该函数接受两个参数：

  • 第一个参数名为 dataFile，类型为 istream_iterator<int>，dataFile 两遍的括号可以忽略。
  • 第二个参数是一个函数指针，该函数不接受参数但返回一个 istream_iterator<int>。

这个和我们最开始想象的完全不一样，而产生分歧的原因就在于 C++ 对代码的解析上：

    只要表达式可以被解析成 函数 ，那么该表达式 就会被编译器解析成函数 ！

想象一下下面这段代码，相信很多人都写出来过，但是它能编译么？

 7: class Widget
 8: {
 9: public:
10:     Widget(){}
11:     virtual ~Widget(){}
12:     void Show(){}
13: };
14: 
15: Widget w();
16: w.Show();

上面片段的第 8 行实际上不是声明了一个 Widget 对象，而是声明了一个用来返回 Widget 对象的函数，第 9 行自然也就出错了。

理解了上面的内容，也就可以想想怎么解决开始时提出的问题了：给形参声明加上括号不合法，但给函数调用的实参加括号是合法的，通过适当的添加括号，问题得以解决：

1: list<int> data((istream_iterator<int>(dataFile)),
2:                istream_iterator<int>());

这里第一个参数周围添加了多余的括号，假设编译器仍然认为 data() 是一个函数声明，则第一个形参周围就被添加了括号，这是一个非法的行为，所以编译器会丢掉这种可能，转而匹配下一种可能得匹配，认为该表达式声明了一个 list<data> 的变量，并调用适当的区间函数（Item 5）来进行初始化。

但并非所有的编译器都支持这种匿名对象，如果编译器不支持，我们需要下面的这种显示的写法：

1: ifstream dataFile("ints.dat");
2: istream_iterator<int> dataBegin(dataFile);
3: istream_iterator<int> dataEnd;
4: list<int> data(dataBegin, dataEnd);

这种写法应该通用。

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