Can My Custom Class Be Used as a Key in a C std::map?

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:

1. Copyability and Assignability: Keys must be copyable and assignable, allowing for efficient duplication and modification within the map.
2. 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.

The above is the detailed content of Can My Custom Class Be Used as a Key in a C std::map?. For more information, please follow other related articles on the PHP Chinese website!