Using Arbitrary Classes as Keys in std::map
std::map, a fundamental component of the C Standard Template Library, efficiently associates key-value pairs. When using custom classes as keys, certain criteria must be met to ensure valid operation.
Requirements for Key Classes
For a class to serve as a valid key in std::map, it must fulfill the following requirements:
- Copyability and Assignability: Keys must be copyable and assignable, allowing for efficient duplication and modification within the map.
-
Ordering: The ordering of elements within std::map is determined by the third template argument or the constructor argument used. By default, this defaults to std::less
, which utilizes the < operator. However, custom comparison operators can be defined to specify alternative ordering criteria.
Defining Custom Comparison Operators
If the default ordering does not meet your requirements, you can define a custom comparison operator as a functional object. This operator must implement a strict ordering, meaning that for any two keys a and b, the following conditions must hold:
- CmpMyType()(a, b) == true implies CmpMyType()(b, a) == false
- CmpMyType()(a, b) == false and CmpMyType()(b, a) == false implies a == b
By fulfilling these requirements, custom classes can be seamlessly used as keys in std::map, enabling efficient ordering and retrieval of associated values.
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