error with the declaration of enum_问答_开发者

I have a very simple c code:

         #include<stdio.h>
        int main()
        {
          enum boolean{true,false};
          boolean bl=false;
          if(bl==false)
             printf("This is the false value of boool\n");
         boolean bl1=true;
          if(bl1==true)
           {
            printf("This is the true value of boool\n");
           }
    return 0;
   }

i was just trying to use enum type variable .but it is giving following error:

tryit4.c:5: error: ‘boolean’ undeclared (first use in this function)
tryit4.c:5: error: (Each undeclared identifier is reported only once
tryit4.c:5: error: for each function it appears in.)
tryit4.c:5: error: expected ‘;’ before ‘bl’
tryit4.c:6: error: ‘bl’ undeclared 开发者_JAVA百科(first use in this function)
tryit4.c:8: error: expected ‘;’ before ‘bl1’
tryit4.c:9: error: ‘bl1’ undeclared (first use in this function)

I don't see any reason for it. Can you please explain what could be the reason for it?

In C, there are two (actually more, but i keep it at this) kind of namespaces: Ordinary identifiers, and tag identifiers. A struct, union or enum declaration introduces a tag identifier:

enum boolean { true, false };
enum boolean bl = false;

The namespace from which the identifier is chosen is specified by the syntax around. Here, it is prepended by a enum. If you want to introduce an ordinary identifier, put it inside a typedef declaration

typedef enum { true, false } boolean;
boolean bl = false;

Ordinary identifiers don't need special syntax. You may declare a tag and ordinary one too, if you like.

When you declare enum boolean { true, false }, you declare a type called enum boolean. That the name you'll have to use after that declaration: enum boolean, not just boolean.

If you want a shorter name (like just boolean), you'll have to define it as an alias for the original full name

typedef enum boolean boolean;

If you wish, you can declare both the enum boolean type and the boolean alias on one declaration

typedef enum boolean { true, false } boolean;

You have to declare the variables to be of type enum boolean, not just boolean. Use typedef, if you find writing enum boolean b1 = foo(); cumbersome.

It would really be a good idea to define your enum like this:

typedef enum {
  False,
  True,
} boolean;

A couple of reasons:

true and false (lowercase) are likely reserved words
false being 1 and true being 0 can cause you logic problems later

You declare the enum, but not the type. What you want is

typedef enum{false, true} boolean;  // false = 0 is expected by most programmers

There are still multiple problems with this:
* true and false are reserved words in many C compilers
* explicitly using true and false goes against the general practice of Boolean expressions in C, where zero means false and anything non-zero means true. For example:

int found = (a == b);

Edit: This works with gcc 4.1.2:

[wally@zf ~]$ ./a.out
This is the false value of boool
This is the true value of boool
[wally@zf ~]$ cat t2.c
#include<stdio.h>
int main()
{
        typedef enum {true,false} boolean;
        boolean bl=false;
        if(bl==false)
                printf("This is the false value of boool\n");
        boolean bl1=true;
        if(bl1==true)
        {
                printf("This is the true value of boool\n");
        }
        return 0;
}

Like previous answers demonstrate, use typedef: