Is is standard practice to have the same include derivate in a header and source file?

Question

When working with say, the standard strings class in c++. Do you need to have #include<string> on both the header and source file?

can you not just have #include<string> only on the header file?

Answer 1

Including on the header file may indicate a dependency at the class definition level (for example you are accepting or returning Strings). In this case you have to put the include in the header; however if you are only using Strings in your implementation you may need to include only on the source file.