使用默认构造函数一个易犯的错误

初级 C++ 程序员常犯的一个错误是，采用以下方式声明一个用默认构造函数初始化的对象：

     // oops! declares a function, not an object
     Sales_item myobj();

The declaration of myobj compiles without complaint. However, when we try to use myobj

编译 myobj 的声明没有问题。然而，当我们试图使用 myobj 时

     Sales_item myobj();   // ok: but defines a function, not an object
     if (myobj.same_isbn(Primer_3rd_ed))   // error: myobj is a function

the compiler complains that we cannot apply member access notation to a function! The problem is that our definition of myobj is interpreted by the compiler as a declaration of a function taking no parameters and returning an object of type Sales_itemhardly what we intended! The correct way to define an object using the default constructor is to leave off the trailing, empty parentheses:

编译器会指出不能将成员访问符号用于一个函数！问题在于 myobj 的定义被编译器解释为一个函数的声明，该函数不接受参数并返回一个 Sales_item 类型的对象——与我们的意图大相径庭！使用默认构造函数定义一个对象的正确方式是去掉最后的空括号：

     // ok: defines a class object ...
     Sales_item myobj;

On the other hand, this code is fine:

另一方面，下面这段代码也是正确的：

     // ok: create an unnamed, empty Sales_itemand use to initialize myobj
     Sales_item myobj = Sales_item();

Here we create and value-initialize a Sales_item object and to use it to initialize myobj. The compiler value-initializes a Sales_item by running its default constructor.

在这里，我们创建并初始化一个 Sales_item 对象，然后用它来按值初始化 myobj。编译器通过运行 Sales_item 的默认构造函数来按值初始化一个 Sales_item。