Scoped enums

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In this section we present a technique for scoping enum values. While the next standard C++0x will bring scoped enums into the language itself, meanwhile we need a convenient work-around.

Problem

Consider the following code:

enum LoopType
{
    None,
    Forward,
    Backward,
    PingPong
};

enum Direction
{
    Forward,
    Backward,
    Left,
    Right
};

void setLoopType(LoopType loopType)
{
    // ...
}

void move(Direction direction)
{
    // ...
}

int main()    
{
    setLoopType(Forward);
    move(Backward);

    return 0;
}

This code won't compile because the enum values collide in identifiers. This is because the enum values are not scoped inside LoopType and Direction. Instead, the enum values reserve identifiers from the surrounding namespace.

Solution

The solution is to wrap enums inside a class, in effect offering the enum values their own namespace:

class LoopType
{
public:
    enum Enum
    {
        None,
        Forward,
        Backward,
        PingPong
    };
};

class Direction
{
public:
    enum Enum
    {
        Forward,
        Backward,
        Left,
        Right
    };
};

void setLoopType(LoopType::Enum loopType)
{
    // ...
}

void move(Direction::Enum direction)
{
    // ...
}

int main()    
{
    setLoopType(LoopType::Forward);
    move(Direction::Backward);

    return 0;
}

The extra effort you need to do now is to rewrite the types of the enums as LoopType::Enum and Direction::Enum instead of just LoopType and Direction. Additionally, you always have to be explicit with the enum values, qualifying the enum of which you are referring to (e.g. LoopType::Forward). I find the the enum type name (e.g. LoopType::Enum) nice because it is in symmetry with the enum value names (e.g. LoopType::Forward) as if saying 'replace Enum with your desired enum value'.