What are the scenarios where pairs are used in C++?

C++中pair的应用场景有很多，一些常见的应用场景包括：

1. Combine two different types of data together. The pair can be used to combine two different types of data into one object, making it easy to return multiple values in a function or store multiple types of data in a container.
2. Store key-value pairs in a container. Pairs can be used to store key-value pairs, such as storing associated key-value pairs in map or unordered_map containers.
3. As a return value of a function, a pair can be used to easily return multiple values.
4. In algorithms, some functions like sort and min/max can utilize pairs to specify the range for sorting or searching.
5. Pair can be used in custom data structures to store two related pieces of data.

In general, the usage of pairs in C++ is very versatile, as it can be used in various situations where two pieces of data need to be combined.