为什么这个C代码中的结构有两个名称？

Question

我一直在使用"学习核心音频"这本书中的一些示例代码.

typedef struct MySineWavePlayer
{
  AudioUnit outputUnit;
  double startingFrameCount;
} MySineWavePlayer;

为什么此代码中的"MySineWavePlayer"两次？

Answer 1

这是一个常见的问题,通过解释在定义结构时使用typedef而不使用typedef之间的区别来回答.

常见的习语是同时使用: typedef struct X { int x; } X;

它们是不同的定义.为了使讨论更清楚,我将分开句子:

struct S { int x; };
typedef struct S S;

在第一行中,您将S在结构名称空间中定义标识符(而不是在C++意义上).您可以使用它并通过将参数的类型定义为struct S:定义新定义类型的变量或函数参数:

void f( struct S argument ); // struct is required here

The second line adds a type alias S in the global name space and thus allows you to just write:

void f( S argument ); // struct keyword no longer needed

Note that since both identifier name spaces are different, defining S both in the structs and global spaces is not an error, as it is not redefining the same identifier, but rather creating a different identifier in a different place.

To make the difference clearer:

typedef struct S { int x; } T;
void S() {} // correct
//void T() {} // error: symbol T already defined as an alias to 'struct S'

You can define a function with the same name of the struct as the identifiers are kept in different spaces, but you cannot define a function with the same name as a typedef as those identifiers collide.

In C++, it is slightly different as the rules to locate a symbol have changed subtly. C++ still keeps the two different identifier spaces, but unlike in C, when you only define the symbol within the class identifier space, you are not required to provide the struct/class keyword:

 // C++
 struct S { int x; }; // S defined as a class
 void f( S a ); // correct: struct is optional

What changes are the search rules, not where the identifiers are defined. The compiler will search the global identifier table and after S has not been found it will search for S within the class identifiers.

The code presented before behaves in the same way:

typedef struct S { int x; } T;
void S() {} // correct [*]
//void T() {} // error: symbol T already defined as an alias to 'struct S'

After the definition of the S function in the second line, the struct S cannot be resolved automatically by the compiler, and to create an object or define an argument of that type you must fall back to including the struct keyword:

// previous code here...
int main() {
    S(); 
    struct S s;
}

source

Answer 2

在这种情况下实际上不需要第一个，第二个用于typedef如果你写这个

struct MySineWavePlayer
{
  AudioUnit outputUnit;
  double startingFrameCount;
};

那么你需要做这样的事情来声明一个 struct 的实例

struct MySineWavePlayer mySineWavePlayer;

然后你可以这样做

typedef struct MySineWavePlayer MySineWavePlayer;

然后声明将成为

MySineWavePlayer mySineWavePlayer;

所以原来的

typedef struct MySineWavePlayer
{
  AudioUnit outputUnit;
  double startingFrameCount;
} MySineWavePlayer;

是这些的组合。

你甚至可以typedef为匿名结构做

typedef struct
{
  AudioUnit outputUnit;
  double startingFrameCount;
} MySineWavePlayer;

所以简短的回答是第一个MySineWavePlayer是d 类型的名称，struct而第二个是typedefd 类型的名称。