Is using enum for integer bit oriented operations in C++ reliable/safe?

Question

Consider the following (simplified) code:

enum eTestMode
{
    TM_BASIC        = 1,    // 1 << 0
    TM_ADV_1        = 1 << 1,
    TM_ADV_2        = 1 << 2
};

...

int m_iTestMode;    // a "bit field"

bool isSet( eTestMode tsm )
{ 
    return ( (m_iTestMode & tsm) == tsm );
}

void setTestMode( eTestMode tsm )
{
    m_iTestMode |= tsm;
}

Is this reliable, safe and/or good practice? Or is there a better way of achieving what i want to do apart from using const ints instead of enum? I would really prefer enums, but code reliability is more important than readability.

Answer 1

I can't see anything bad in that design.

However, keep in mind that enum types can hold unspecified values. Depending on who uses your functions, you might want to check first that the value of tsm is a valid enumeration value.

Since enums are integer values, one could do something like:

eTestMode tsm = static_cast<eTestMode>(17); // We consider here that 17 is not a valid value for your enumeration.

However, doing this is ugly and you might just consider that doing so results in undefined behavior.

Answer 2

There is no problem. You can even use an variable of eTestMode (and defines bit manipulation for that type) as it is guaranteed to hold all possible values in that case.

Answer 3

See also What is the size of an enum in C?

For some compilers (eg VC++) this non-standard width specifier can be used:

enum eTestMode : unsigned __int32
{ 
    TM_BASIC        = 1,    // 1 << 0 
    TM_ADV_1        = 1 << 1, 
    TM_ADV_2        = 1 << 2 
};

Answer 4

使用枚举来表示位模式，掩码和标志并不总是一个好主意，因为枚举通常会提升为有符号整数类型，而对于基于位的操作， 无符号类型几乎总是更可取。