what is the size of an enum type data in C++?

This is a C++ interview test question not homework. 

#include <iostream>
using namespace std;
enum months_t { january, february, march, april, may, june, july, august, september,
october, november, december} y2k;


int main ()
{
cout << "sizeof months_t is  " << sizeof(months_t) << endl;
cout << "sizeof y2k is  " << sizeof(y2k) << endl;
enum months_t1 { january, february, march, april, may, june, july, august,
september, october, november, december} y2k1;
cout << "sizeof months_t1 is  " << sizeof(months_t1) << endl;
cout << "sizeof y2k1 is  " << sizeof(y2k1) << endl;
}

Output:

sizeof months_t is 4
sizeof y2k is 4
sizeof months_t1 is 4
sizeof y2k1 is 4

Why is the size of all of these 4 bytes? Not 12 x 4 = 48 bytes?
I know union elements occupy the same memory location, but this is an enum.

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最佳答案

In your compiler, the size is four bytes because the enum is stored as an int. With only 12 values, you really only need 4 bits, but 32 bit machines process 32 bit quantities more efficiently than smaller quantities.

0 0 0 0  January
0 0 0 1  February
0 0 1 0  March
0 0 1 1  April
0 1 0 0  May
0 1 0 1  June
0 1 1 0  July
0 1 1 1  August
1 0 0 0  September
1 0 0 1  October
1 0 1 0  November
1 0 1 1  December
1 1 0 0  ** unused **
1 1 0 1  ** unused **
1 1 1 0  ** unused **
1 1 1 1  ** unused **

Without enums, you might be tempted to use raw integers to represent the months. That would work and be efficient, but it would make your code hard to read. With enums, you get efficient storage and readability.

Other compilers could use a byte, int16, uint16 int or uint as long as the variable can contain all the values of the enum.

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Because it's the size of an instance of the type - presumably enum values are stored as (32-bit / 4-byte) ints here.

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An enum is nearly an integer. To simplify a lot

enum yourenum { a, b, c };

is almost like

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

Of course, it is not really true. I'm trying to explain that enum are some kind of coding...

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An enum is kind of like a typedef for the int type (kind of).

So the type you've defined there has 12 possible values, however a single variable only ever has one of those values.

Think of it this way, when you define an enum you're basically defining another way to assign an int value.

In the example you've provided, january is another way of saying 0, feb is another way of saying 1, etc until december is another way of saying 11.

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视情况而定。标准只要求它足够大，可以容纳所有的值，所以形式上像enum foo { zero, one, two };这样的枚举只需要一个字节大。然而，大多数实现使这些枚举与整数一样大，（在现代硬件上更快。此外，它还需要与C兼容，在C中，枚举基本上被美化为整数）。但是，请注意，C++允许初始值设定项在int范围之外的枚举，并且对于这些枚举，大小当然也会更大。例如，如果您有enum bar { a, b = 1LL << 35 };，那么即使在具有32位整数的系统上，您的枚举也将大于32位（很可能是64位）（请注意，在C中不允许使用该枚举）。

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This is a C++ interview test question not homework.

Then your interviewer needs to refresh his recollection with how the C++ standard works. And I quote:

For an enumeration whose underlying type is not fixed, the underlying type is an integral type that can represent all the enumerator values defined in the enumeration.

The whole "whose underlying type is not fixed" part is from C++11, but the rest is all standard C++98/03. In short, the sizeof(months_t) is not 4. It is not 2 either. It could be any of those. The standard does not say what size it should be; only that it should be big enough to fit any enumerator.

why the all size is 4 bytes ? not 12 x 4 = 48 bytes ?

Because enums are not variables. The members of an enum are not actual variables; they're just a semi-type-safe form of #define. They're a way of storing a number in a reader-friendly format. The compiler will transform all uses of an enumerator into the actual numerical value.

Enumerators are just another way of talking about a number. january is just shorthand for 0. And how much space does 0 take up? It depends on what you store it in.

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With my now ageing Borland C++ Builder compiler enums can be 1,2 or 4 bytes, although it does have a flag you can flip to force it to use ints.

I guess it's compiler specific.

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I like the explanation From EdX (Microsoft: DEV210x Introduction to C++) for a similar problem:

"The enum represents the literal values of days as integers. Referring to the numeric types table, you see that an int takes 4 bytes of memory. 7 days x 4 bytes each would require 28 bytes of memory if the entire enum were stored but the compiler only uses a single element of the enum, therefore the size in memory is actually 4 bytes."


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