C转发声明导致未知的类型错误:我如何得到类型、结构等的顺序？

Question

我知道有类似的帖子，但我很难掌握它的诀窍:我有一个.c和一个.h文件：

graph.c

#ifndef graph
#define graph

/*1. standard headers*/
#include <stdbool.h>

/*2. user-def. headers*/
//#include ".."

/*3. global macros*/
//#define GLOB_MACRO … 

/*4. global type decls.*/

struct graph;
typedef struct graph graph_t;

graph_t* graph_create(int n);
void graph_free(graph_t *g);

void graph_insert_edge(graph_t *g, int source, int target, double weight);
void graph_remove_edge(graph_t *g, int source, int target);

bool graph_has_edge(graph_t *g, int source, int target);
double graph_get_weight(graph_t *g, int source, int target);

void graph_print(graph_t *g);

/*5. glob. var. decls.*/
//extern … glob_var; 

/*6. glob. func. decls*/
//[extern] … glob_func(…);

#endif

graph_m.c

// Driver Code
int main()
{
    printf("test");
 
    return 0;
}

但是我得到了这些编译器错误：

gcc  -MM graph_m.c > .depend
gcc -pedantic -Wall -Wextra -Wvla -std=c11 -ggdb3    -c -o graph_m.o graph_m.c
In file included from graph_m.c:14:
graph.h:15:13: error: expected '{' before ';' token
   15 | struct graph;
      |             ^
graph.h:16:22: warning: useless storage class specifier in empty declaration
   16 | typedef struct graph graph_t;
      |                      ^~~~~~~
graph.h:18:1: error: unknown type name 'graph_t'; use 'struct' keyword to refer to the type
   18 | graph_t* graph_create(int n);
      | ^~~~~~~
      | struct
graph.h:19:17: error: unknown type name 'graph_t'; did you mean 'graph'?
   19 | void graph_free(graph_t *g);
      |                 ^~~~~~~
      |                 graph
graph.h:21:24: error: unknown type name 'graph_t'; did you mean 'graph'?
   21 | void graph_insert_edge(graph_t *g, int source, int target, double weight);
      |                        ^~~~~~~
      |                        graph
graph.h:22:24: error: unknown type name 'graph_t'; did you mean 'graph'?
   22 | void graph_remove_edge(graph_t *g, int source, int target);
      |                        ^~~~~~~
      |                        graph
graph.h:24:21: error: unknown type name 'graph_t'; did you mean 'graph'?
   24 | bool graph_has_edge(graph_t *g, int source, int target);
      |                     ^~~~~~~
      |                     graph
graph.h:25:25: error: unknown type name 'graph_t'; did you mean 'graph'?
   25 | double graph_get_weight(graph_t *g, int source, int target);
      |                  graph
graph_m.c:24:1: warning: unnamed struct/union that defines no instances
   24 | } graph;
      | ^
make: *** [<builtin>: graph_m.o] Error 1
root@8c9bf05a5390:/home/swo3/src/graph# In file included from graph_m.c:14:
graph.h:15:13: error: expected '{' before ';' token
   15 | struct graph;
      |             ^
graph.h:16:22: warning: useless storage class specifier in empty declaration
   16 | typedef struct graph graph_t;
      |                      ^~~~~~~
graph.h:18:1: error: unknown type name 'graph_t'; use 'struct' keyword to refer to the type
   18 | graph_t* graph_create(int n);
      | ^~~~~~~
      | struct
make: *** [<builtin>: graph_m.o] Error 1union that defines no instances?e weight);

我是不是弄错命令了？我在运行中有ty胡枝子和struct，对于头文件的实际实现有不同的文件名，其中我想在后台为graph.h、graph_m.c和另一个数据结构做两个数据结构。

Answer 1

宏名与其他名称发生冲突。

你先从这个开始：

#define graph

然后你就有了这个

struct graph;

由于宏做了直接的令牌替换，在预处理后，上面的行变成：

struct ;

这是无效的。

您需要更改宏名，这样就不会发生冲突。按照惯例，宏通常是大写的，头保护宏通常以_H结尾。因此，将前两行更改为：

#ifndef GRAPH_H
#define GRAPH_H

Answer 2

在#define graph之后，宏处理将struct graph;替换为struct ;。

为预处理器保护使用不同的标识符，如graph_h。