All programming languages you would have learned so far store their variables in memory. But in Solidity, variables are stored in the memory and the filesystem depending on the context.
Depending on the context, there is always a default location. But for complex data types, such as strings, arrays, and structs, it can be overridden by appending either storage or memory to the type. The default for function parameters (including return parameters) is memory, the default for local variables is storage. and the location is forced to storage, for state variables (obviously).
Data locations are important because they change how assignments behave:
- Assignments between storage variables and memory variables always create an independent copy. But assignments from one memory-stored complex type to another memory-stored complex type do not create a copy.
- Assignment to a state variable (even from other state variables) always creates an independent copy.
- You cannot assign complex types stored...