Can Any Class Serve as a Key for std::map?

Keys for std::map: Requirements and Implementation

When utilizing std::map for mapping objects of different classes, the class you intend to use as a key may not satisfy the necessary requirements for valid keys. std::map organizes its contents using an ordering mechanism, which raises the question of whether any arbitrary class can serve as a key or if specific requirements exist.

The key for std::map must adhere to the following requirements:

• Copiability and Assignability: The key class must be able to be copied and assigned.

The map's ordering is controlled by the third template argument or the constructor argument, which defaults to std::less. By default, std::less leverages the < operator. However, you have the flexibility to define your own comparison operator:

struct CmpMyType
{
    bool operator()( MyType const& lhs, MyType const& rhs ) const
    {
        // Implementation of comparison logic
    }
};

Ensure that your comparison operator defines a strict ordering. If CmpMyType()( a, b ) returns true, CmpMyType()( b, a ) must return false. When both return false, the elements are considered equivalent and belong to the same equivalence class.

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