为什么在文件范围内初始化枚举类型变量时出现错误？

Question

I am unable to understand the situation when I try to initialize the enum variable as global, ie, outside of any scope. For example if I try to compile below code:

#include <stdio.h>

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
} card;

card = club;
int main() 
{ 
    printf("Size of enum variable = %d bytes", sizeof(card));   
    return 0;
}

Following compilation errors happen:

prog.c:9:1: warning: data definition has no type or storage class
card = club;
^
prog.c:9:1: warning: type defaults to 'int' in declaration of 'card' [Wimplicit-int]
prog.c:9:1: error: conflicting types for 'card'
prog.c:8:3: note: previous declaration of 'card' was here
} card;
  ^

But when I put the initialization within the main() scope , no error occurs like the code below:

#include <stdio.h>

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
} card;

int main() 
{
    card = club;
    printf("Size of enum variable = %d bytes", sizeof(card));   
    return 0;
}

Output is :

Size of enum variable = 4 bytes

Answer 1

Your first declares a global variable named club of type enum suit , however you cannot initialize a global variable on another line outside of a function - you must do it on the same line, like so:

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
} card = club;

I feel your code would be more readable if you moved the enum club definition away from the variable declaration:

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
};

enum suit card = club;

The extra enum keyword is because C (unlike C++) has a different namespace for each kind of type - so you can have enum foo and struct foo in the same project without the names colliding because enum and struct type names exist in different namespaces.

(I'm personally not a fan of using typedef - but feel free to go with that if you choose).

Answer 2

This happens because

  card = club;

is an assignment statement , which needs to be in block scope (some function body) not in file scope, not an initialization .

If you use initialization , it'll work.

Also note the extra changes

• int main() should be int main(void)
• sizeof yields a size_t , so you should use %zu format specifier to print the result.

Example

#include <stdio.h>

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
} card = club;

int main(void) 
{ 
    printf("Size of enum variable = %zu bytes", sizeof(card));   
    return 0;
}