Enum declaration

Question

enum Abc
{
i = 0,
j = 1
};

enum {idd= 100 };

I am aware about first syntax of enum declaration. What is use of second declaration? I mean what is name of enum in it and why it is allowed in c++?

Answer 1

Personally, I've never used enum without a name, but I found some posts about this:

cppreference( https://en.cppreference.com/w/cpp/language/enum ): The name of an unscoped enumeration (enum without class or struct) may be omitted: such declaration only introduces the enumerators into the enclosing scope:

enum { a, b, c = 0, d = a + 2 }; // defines a = 0, b = 1, c = 0, d = 2

A Stack Overflow post about 'Anonymous enum' can be found here: https://stackoverflow.com/questions/7147008/the-usage-of-anonymous-enums/7147049

Answer 2

CarrieForle so, technically, what's the difference between using:

#define y 100

and:

enum {y=100};

?

Answer 3

The elements of enum are of compile-time constant. This applies to both unscoped and scoped.

In the following example:
#include <iostream>
using namespace std;

enum A {a, b=100};
enum {x, y=100};
enum class C {apple};

int main() {
cout << &(A::a);
cout << &x;
cout << &(C::apple);
return 0;
}

3 Errors occurred from 3 cout statements are all that the address operator requires lvalue operand. This means the elements don't have addresses, which apply to the compile-time constant behavior. They don't consume any memory.

Answer 4

visph
One is a marco, can define anything other than a 'constant integer', and the other is a (seris of) constant values. Though if you look at the result, they appears to be the same. #define is just another way to achieve it.

But how about:

#define a 0
#define b 1
#define c 2
#define d 3

vs

enum {a, b, c, d};

Interestingly enough, there is a comparison between these 'in C' in Stack Overflow, with one more opponent 'static const'.

https://stackoverflow.com/questions/1674032/static-const-vs-define-vs-enum

Answer 5

Thanks CarrieForle ... I have also not used it but came across same in MFC examples.

Also I got point that enum value cannot be accidently changed unlike int , but don't understand how it doesnot consume memory..

Refer below code and all three enum member including unnamed enum prints size as 4 same as int

enum A{i=0};
enum {idd=12};

int main()
{
cout << sizeof(A) << endl;
cout << sizeof(i) << endl;
cout << sizeof(idd) << endl;

//gives error
//idd= 13;

return 0;
}