C

enum stuff q;
enum stuff {a, b=-4, c, d=-2, e, f=-3, g} s;

Declaration which acts as a tentative definition of a signed integer s with complete type and declaration which acts as a tentative definition of signed integer q with incomplete type in the scope (which resolves to the complete type in the scope because the type definition is present anywhere in the scope) (like any tentative definition, the identifiers q and s can be redeclared with the incomplete or complete version of the same type int or enum stuff multiple times but only defined once in the scope i.e. int q = 3; and can only be redefined in a subscope, and only usable after the definition). Also you can only use the complete type of enum stuff once in the scope because it acts as a type definition.

A compiler enumeration type definition for enum stuff is also made present at file scope (usable before and below) as well as a forward type declaration (the type enum stuff can have multiple declarations but only one definition/completion in the scope and can be redefined in a subscope). It also acts as a compiler directive to substitute a with rvalue 0, b with -4, c with 5, d with -2, e with -3, f with -1 and g with -2 in the current scope. The enumeration constants now apply after the definition until the next redefinition in a different enum which cannot be on the same scope level.

typedef enum bool {false, true} bool;

//this is the same as 
enum bool {false, true};
typedef enum bool bool;

//or
enum bool {false, true};
typedef unsigned int bool;

//remember though, bool is an alias for _Bool if you include stdbool.h. 
//and casting to a bool is the same as the !! operator 

The tag namespace shared by enum, struct and union is separate and must be prefixed by the type keyword (enum, struct or union) in C i.e. after enum a {a} b, enum a c must be used and not a c. Because the tag namespace is separate to the identifier namespace, enum a {a} b is allowed but enum a {a, b} b is not because the constants are in the same namespace as the variable identifiers, the identifier namespace. typedef enum a {a,b} b is also not allowed because typedef-names are part of the identifier namespace.

enum bool类型和常量在C中遵循以下模式:

+--------------+-----+-----+-----+
|   enum bool  | a=1 |b='a'| c=3 |  
+--------------+-----+-----+-----+
| unsigned int | int | int | int |  
+--------------+-----+-----+-----+

+--------------+-----+-----+-----+
|   enum bool  | a=1 | b=-2| c=3 |  
+--------------+-----+-----+-----+
|      int     | int | int | int |  
+--------------+-----+-----+-----+

+--------------+-----+---------------+-----+
|   enum bool  | a=1 |b=(-)0x80000000| c=2 |
+--------------+-----+---------------+-----+
| unsigned int | int |  unsigned int | int |
+--------------+-----+---------------+-----+

+--------------+-----+---------------+-----+
|   enum bool  | a=1 |b=(-)2147483648| c=2 |
+--------------+-----+---------------+-----+
| unsigned int | int |  unsigned int | int |
+--------------+-----+---------------+-----+

+-----------+-----+---------------+------+
| enum bool | a=1 |b=(-)0x80000000| c=-2 |
+-----------+-----+---------------+------+
|    long   | int |      long     |  int |
+-----------+-----+---------------+------+

+-----------+-----+---------------+------+
| enum bool | a=1 | b=2147483648  | c=-2 |
+-----------+-----+---------------+------+
|    long   | int |      long     |  int |
+-----------+-----+---------------+------+

+-----------+-----+---------------+------+
| enum bool | a=1 | b=-2147483648 | c=-2 |
+-----------+-----+---------------+------+
|    int    | int |      int      |  int |
+-----------+-----+---------------+------+

+---------------+-----+---------------+-----+
|   enum bool   | a=1 | b=99999999999 | c=1 |
+---------------+-----+---------------+-----+
| unsigned long | int | unsigned long | int |
+---------------+-----+---------------+-----+

+-----------+-----+---------------+------+
| enum bool | a=1 | b=99999999999 | c=-1 |
+-----------+-----+---------------+------+
|    long   | int |      long     |  int |
+-----------+-----+---------------+------+

这在C中编译很好:

#include <stdio.h>
enum c j;
enum c{f, m} p;
typedef int d;
typedef int c;
enum c j;
enum m {n} ;
int main() {
  enum c j;
  enum d{l};
  enum d q; 
  enum m y; 
  printf("%llu", j);
}

C++

在c++中，枚举可以有类型

enum Bool: bool {True, False} Bool;
enum Bool: bool {True, False, maybe} Bool; //error

在这种情况下，常量和标识符都具有相同的类型bool，如果一个数字不能用该类型表示，就会发生错误。也许= 2，这不是bool类型。此外，True、False和Bool不能小写，否则它们将与语言关键字冲突。枚举也不能有指针类型。

在c++中枚举的规则是不同的。

#include <iostream>
c j; //not allowed, unknown type name c before enum c{f} p; line
enum c j; //not allowed, forward declaration of enum type not allowed and variable can have an incomplete type but not when it's still a forward declaration in C++ unlike C
enum c{f, m} p;
typedef int d;
typedef int c; // not allowed in C++ as it clashes with enum c, but if just int c were used then the below usages of c j; would have to be enum c j;
[enum] c j;
enum m {n} ;
int main() {
  [enum] c j;
  enum d{l}; //not allowed in same scope as typedef but allowed here 
  d q;
  m y; //simple type specifier not allowed, need elaborated type specifier enum m to refer to enum m here
  p v; // not allowed, need enum p to refer to enum p
  std::cout << j;
}

c++中的enum变量不再只是无符号整数等，它们也是enum类型，只能在enum中赋值常量。然而，这是可以抛弃的。

#include <stdio.h>
enum a {l} c;
enum d {f} ;
int main() {
  c=0; // not allowed;
  c=l;
  c=(a)1;
  c=(enum a)4;
  printf("%llu", c); //4
}

枚举类

Enum struct与Enum类相同

#include <stdio.h>
enum class a {b} c;
int main() {
  printf("%llu", a::b<1) ; //not allowed
  printf("%llu", (int)a::b<1) ;
  printf("%llu", a::b<(a)1) ;
  printf("%llu", a::b<(enum a)1);
  printf("%llu", a::b<(enum class a)1) ; //not allowed 
  printf("%llu", b<(enum a)1); //not allowed
}

范围解析操作符仍然可以用于非范围枚举。

#include <stdio.h>
enum a: bool {l, w} ;
int main() {
  enum a: bool {w, l} f;
  printf("%llu", ::a::w);
}

但是因为w不能被定义为作用域中的其他东西，所以::w和::a::w之间没有区别

如前所述，您的代码没有任何问题。你确定你没做过类似的事吗

int strategy;
...
enum {RANDOM, IMMEDIATE, SEARCH} strategy;

错误消息指向哪些行?当它说“以前的‘战略’宣言在这里”时，“这里”是什么?它显示了什么?

@ThoAppelsin in his comment to question posted is right. The code snippet posted in the question it is valid and with no errors. The error you have must be because other bad syntax in any other place of your c source file. enum{a,b,c}; defines three symbolic constants (a, b and c) which are integers with values 0,1 and 2 respectively, but when we use enum it is because we don't usually care about the specific integer value, we care more about the meaning of the symbolic constant name. This means you can have this:

#include <stdio.h>
enum {a,b,c};
int main(){
  printf("%d\n",b);
  return 0;
}

这个输出是1。

这也是有效的:

#include <stdio.h>
enum {a,b,c};
int bb=b;
int main(){
  printf("%d\n",bb);
  return 0;
}

并将输出与之前相同的结果。

如果你这样做:

enum {a,b,c};
enum {a,b,c};

你会有一个错误，但是如果你这样做:

enum alfa{a,b,c};
enum alfa;

你不会有任何错误。

你可以这样做:

enum {a,b,c};
int aa=a;

aa是一个值为0的整型变量。但你也可以这样做:

enum {a,b,c} aa= a;

和将具有相同的效果(即aa是一个值为0的int)。

你还可以这样做:

enum {a,b,c} aa= a;
aa= 7;

aa将是int，值为7。

因为你不能使用enum重复符号常量定义，就像我之前说过的，如果你想使用enum声明int变量，你必须使用标签:

enum tag1 {a,b,c};
enum tag1 var1= a;
enum tag1 var2= b;

typedef的使用是为了保证你不用每次都用枚举tag1来定义变量。使用typedef，你可以输入Tag1:

typedef enum {a,b,c} Tag1;
Tag1 var1= a;
Tag1 var2= b;

你还可以有:

typedef enum tag1{a,b,c}Tag1;
Tag1 var1= a;
enum tag1 var2= b;

最后要说的是，因为我们谈论的是定义的符号常量，所以在使用enum时最好使用大写字母，例如:

enum {A,B,C};

而不是

enum {a,b,c};

声明一个枚举变量是这样的:

enum strategy {RANDOM, IMMEDIATE, SEARCH};
enum strategy my_strategy = IMMEDIATE;

但是，你可以使用typedef来缩短变量声明，如下所示:

typedef enum {RANDOM, IMMEDIATE, SEARCH} strategy;
strategy my_strategy = IMMEDIATE;

有一个命名约定来区分类型和变量是一个好主意:

typedef enum {RANDOM, IMMEDIATE, SEARCH} strategy_type;
strategy_type my_strategy = IMMEDIATE;

当你说

enum {RANDOM, IMMEDIATE, SEARCH} strategy;

创建一个名为“strategy”的实例变量。这不是一件非常有用的事情——你需要一个类型定义:

typedef enum {RANDOM, IMMEDIATE, SEARCH} StrategyType; 
StrategyType strategy = IMMEDIATE;

我尝试了gcc，并提出了我的需要，我被迫使用最后一种选择，编译出来的错误。

类型定义enum状态{a = 0, b = 1, c = 2}状态;

typedef enum state {a = 0, b = 1, c = 2} state;

typedef enum state old; // New type, alias of the state type.
typedef enum state new; // New type, alias of the state type.

new now     = a;
old before  = b;

printf("State   now = %d \n", now);
printf("Sate before = %d \n\n", before);