c++ - 显式调用 WINAPI ReadFile()

Question

我正在使用 Garmin GPS 设备，并尝试通过 USB 获取与纬度、经度等相关的信息。我在 SDK 中工作并达到了可以提取此信息的程度，所以现在我正在努力将此代码滚动到我正在使用的系统中。

这段代码被放置在一个类中:CGarminUSBEngine，它继承自 CASCIIEngine

我使用代码从设备中提取此信息

ReadFile (gHandle,
  TheNewBuffer,
  MAX_BUFFER_SIZE,
  &theBytesReturned,
  NULL);

我的问题是 CGarminUSBEngine 继承的 CASCIIEngine 具有该功能

BOOL ReadFile(void)

当我尝试使用 5 个参数调用 ReadFile() 以从我的 USB 设备读取时，我收到编译器错误消息“'CASCIIEngine::Readfile':函数不接受 5 个参数”

我还没有弄清楚如何显式调用此 ReadFile() 函数的 WINAPI 版本，甚至不知道为什么需要调用它。在我看来，编译器应该知道 ReadFile 已过载，并且我正在尝试调用 WINAPI 版本，而不是 CASCIIEngine 版本。

我主要想知道如何显式调用 WINAPI 版本，其次我想确切地知道为什么需要这样做。也许它只是缺少一些简单的东西，这会令人沮丧。我感谢任何帮助。

Answer 1

您收到错误是因为编译器正在解析对 CASCIIEngine::Readfile() 的调用，而不是 Windows API 提供的全局 ReadFile() 函数^†.

显然参数的数量不匹配，所以编译器会报错。

要告诉编译器引用全局 ReadFile() 函数，请使用 :: 范围解析运算符。

   ::ReadFile (gHandle, TheNewBuffer, MAX_BUFFER_SIZE, &theBytesReturned, NULL);
// ^^---- Note `::`

---

† 确定解析哪个函数的确切规则在 C++ 标准中有非常详细的规定。针对您的情况，相关条款如下:

3.4.1 Unqualified name lookup [basic.lookup.unqual]

1. In all the cases listed in 3.4.1, the scopes are searched for a declaration in the order listed in each of the respective categories; name lookup ends as soon as a declaration is found for the name. If no declaration is found, the program is ill-formed.

...

8. A name used in the definition of a member function (9.3) of class X following the function’s declarator-id shall be declared in one of the following ways:

• before its use in the block in which it is used or in an enclosing block (6.3), or
• shall be a member of class X or be a member of a base class of X (10.2), or
• if X is a nested class of class Y (9.7), shall be a member of Y, or shall be a member of a base class of Y (this lookup applies in turn to Y’s enclosing classes, starting with the innermost enclosing class), or
• if X is a local class (9.8) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or
• if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the member function definition, in namespace N or in one of N’s enclosing namespaces.

...

9.3.1 Nonstatic member functions [class.mfct.nonstatic]

2. When an id-expression (5.1) that is not part of a class member access syntax (5.2.5) and not used to form a pointer to member (5.3.1) is used in the body of a nonstatic member function of class X or used in the mem-initializer for a constructor of class X, if name lookup (3.4.1) resolves the name in the id-expression to a nonstatic nontype member of class X or of a base class of X, the id-expression is transformed into a class member access expression (5.2.5) using (*this) (9.3.2) as the postfix-expression to the left of the . operator. The member name then refers to the member of the object for which the function is called ...

根据 3.4.1/1 和 3.4.1/8，首先找到 CASCIIEngine 类中的 ReadFile() 声明(第二个要点)，然后全局命名空间中的 ReadFile()(最后一个要点)，因此名称查找解析为成员函数。

这意味着您在类里面对 ReadFile() 的调用实际上被解析为:

(*this).ReadFile(gHandle, TheNewBuffer, MAX_BUFFER_SIZE,
    &theBytesReturned, NULL); // Per 9.3.1/2

所以全局函数甚至没有被考虑用于重载决议，因为它不是成员函数。

当您以上述方式使用 :: 范围解析运算符时，它成为一个限定名称，引用全局命名空间中的名称，因此符合上述规则不适用。