如何将枚举与其第零个值进行比较？

Question

In c, it is not defined by the standard whether enums are signed or unsigned. However, when I try to compare an enum value to the lowest (ie 0) enumeration constant, I get the warning "pointless comparison of unsigned integer with zero." (Compiler is IAR embedded workbench.)

typedef enum
{
    BAR,
    BAZ
} Foo;

//later...
Foo x = (Foo)some_integral_value;
if (x >= BAR) // <- this gives me the warning
    //stuff

I need to check the range of the enum, however, because it is being converted from an integral type. Is there a good way to do this that avoids the warning, which will still work if the compiler decides to change the underlying type?

Answer 1

In C this is a false problem.

• all enumeration constants are always of type int , anyhow
• conversions back and forth the enumeration type work easily with implicit conversion, no explicit conversions (AKA cast) are necessary nor desirable

Now to your example

enum Foo
{
    BAR,
    BAZ
};

//later...
Foo x = (Foo)some_integral_value;

This doesn't even compile, because in C Foo is not defined to be anything, you must use enum Foo , or provide an appropriate typedef , something like

typedef enum Foo Foo;

Perhaps you compile C code with a C++ compiler? In any case, provide a complete example that shows your problem.

Answer 2

It sounds like in C, all enums should have type int , but I'm leaving these suggestions around, since your compiler is either non-standard C, or compiling as C++.

---

If you have control over the definitions of these enums, you could just make them start at 1:

enum Foo
{
    BAR = 1,
    BAZ
};

---

You could also add a single fake negative value to force it to be signed:

enum Foo
{
    NEGATIVE_PLACEHOLDER = -1,
    BAR,
    BAZ,
};

---

In C++11, you can give your enum an explicit underlying type:

enum Foo : int
{
    BAR,
    BAZ
};

See this page , specifically the section that says:

enum name : type { enumerator = constexpr , enumerator = constexpr , ... }

...

2) declares an unscoped enumeration type whose underlying type is fixed

It sounds like the type should always be predictable though:

Values of unscoped enumeration type are implicitly-convertible to integral types. If the underlying type is not fixed, the value is convertible first type from the following list able to hold their entire value range: int , unsigned int , long , unsigned long , long long , or unsigned long long . If the underlying type is fixed, the values can be converted to their promoted underlying type.

So, if your enum fits in an int , it should always use an int .

Answer 3

Brendan's first comment is applicable to C and does answer the question. That is, forcing the value of the first item in the enum to 1 eliminates the compile warning. Whether or not this is a viable answer in the original poster's situation depends upon whether or not the first value has to be zero for some other reason.

ps Sorry for posting a response to the question instead of to the answer I'm commenting on but Stack Overflow won't let me do the latter.