c - 当涉及功能时，结构、 union 等的范围如何工作？

Question

我最近遇到了一些结构，有些事情不是很清楚。假设我有这样一个程序:

struct num print_a(struct num c); 

int main(){
  struct num{
    int a;
    int b;
  }c = {1, 2};
  struct num d;
  d = print_a(c);
}

struct num print_a(struct num c){
  printf("%d", c.a);
  return c;
}

这行得通吗，还是我必须在 main 之外声明 struct num？因为我的函数 print_a 如何“看到”struct num 的含义(它必须返回的类型)，因为它在其范围之外声明？

对不起，如果这个问题很愚蠢

Answer 1

Would this work or do I have to declare struct num outside of main?

那么，为什么不简单地尝试一下呢？

http://ideone.com给出:

Compilation error #stdin compilation error #stdout 0s 9424KB
prog.c: In function ‘main’:
prog.c:11:15: error: type of formal parameter 1 is incomplete
   d = print_a(c);
               ^
prog.c:11:3: error: invalid use of undefined type ‘struct num’
   d = print_a(c);
   ^
prog.c:10:14: warning: variable ‘d’ set but not used [-Wunused-but-set-variable]
   struct num d;
              ^
prog.c: At top level:
prog.c:14:31: error: parameter 1 (‘c’) has incomplete type
 struct num print_a(struct num c){
                               ^
prog.c:14:12: error: return type is an incomplete type
 struct num print_a(struct num c){
            ^~~~~~~
prog.c:14:12: error: conflicting types for ‘print_a’
prog.c:3:12: note: previous declaration of ‘print_a’ was here
 struct num print_a(struct num c);
            ^~~~~~~
prog.c: In function ‘print_a’:
prog.c:16:10: warning: ‘return’ with a value, in function returning void
   return c;
          ^
prog.c:14:12: note: declared here
 struct num print_a(struct num c){
            ^~~~~~~

http://coliru.stacked-crooked.com/给出:

main.cpp: In function 'main':
main.cpp:13:15: error: type of formal parameter 1 is incomplete
   d = print_a(c);
               ^
main.cpp:13:7: error: invalid use of undefined type 'struct num'
   d = print_a(c);
       ^~~~~~~
main.cpp:12:14: warning: variable 'd' set but not used [-Wunused-but-set-variable]
   struct num d;
              ^
main.cpp: At top level:
main.cpp:16:31: error: parameter 1 ('c') has incomplete type
 struct num print_a(struct num c){
                    ~~~~~~~~~~~^
main.cpp:16:12: error: return type is an incomplete type
 struct num print_a(struct num c){
            ^~~~~~~
main.cpp:16:12: error: conflicting types for 'print_a'
main.cpp:5:12: note: previous declaration of 'print_a' was here
 struct num print_a(struct num c);
            ^~~~~~~
main.cpp: In function 'print_a':
main.cpp:18:10: warning: 'return' with a value, in function returning void
   return c;
          ^
main.cpp:16:12: note: declared here
 struct num print_a(struct num c){
            ^~~~~~~

嗯...有点给我们答案:不 - 它不会工作

当您在 main 中定义结构时，它仅在 main 中为人所知。如果你想在 main 之外使用结构，你必须将结构的定义移出 main - 比如:

struct num {
    int a;
    int b;
};

struct num print_a(struct num c); 

int main(){
  struct num c = {1, 2};
  struct num d;
  d = print_a(c);
}

struct num print_a(struct num c){
  printf("%d", c.a);
  return c;
}