c ++何时包含cpp，即使我们有.h文件

Question

I am reading the book Absolute C++ 5th edition. In page 716, I don't really understand why it needs include "pfarray.cpp"

Is include "pfarray.h" not enough?

More specifically, even if we have declarations in .h file but implementations in .cpp files, when we still have to include .cpp file?

Thank you in advance.

Answer 1

When you write #include anything.any_extension , the extension doesn't really matter to the preprocessor. It's really like a "take the contents from that file and paste it into this file" kind of brute mechanism. So you can include anything without error provided that the code inside is legit, and you can name a header file with any extension. So you can even name them with a .txt extension and it wouldn't really matter to the preprocessor.

I would suggest that the practice of including source files is rather confusing, mainly from a build standpoint since it's not very clear whether a source file (cpp, cc, etc) is supposed to be built as a separate object file to link against or #included or both.

Yet it's sometimes done anyways. For example, pfarray.cpp might contain an implementation for a template since templates typically need their full implementation visible at compile time at the site generating the code, and sometimes authors establish a habit of #including files with source file extensions to avoid putting the implementation details into the same header file while uniformly conforming to a style that favors putting all such details into files named with a source file convention.

Another reason this can be done, but I don't think it's the reason it was done in your case, is as a build optimization (see Unity builds). It can sometimes be more efficient to compile and link fewer files, so using #include for source files can be a crude way to fuse them all together into a single build target.

Answer 2

You don't have to.

A translation unit is a group of files with definitions and declarations. Compiling a translation unit, the compiler needs to know everything about declarations and re-parse them again and again. The definitions on the other hand can be compiled just once and reused for another units.

A translation unit can be separated in .h and .cpp files. You should put the declarations in .h and definitions in .cpp files to obey one definition rule. This approach also reduces compilation time.

Writing template-d classes and functions (without specialization), some coders (a bad habit in my opinion) will put the implementations in the .cpp files and they have to include them at the end of its corresponding .h file or in a .cpp file which needs them. It's just confusing. A better naming convention is to rename these type of .cpp files to .impl.cpp and including them at the end of its .h file.