Pros & Cons of putting all code in Header files in C++?

Question

You can structure a C++ program so that (almost) all the code resides in Header files. It essentially looks like a C# or Java program. However, you do need at least one

Answer 1

You are right to say that your solution works. It may even have no cons for your current project and developing environment.

But...

As others stated, putting all your code in header files forces a full compilation every time you change one line of code. This may not be an issue yet but your project may grow large enough to the point compilation time will be an issue.

Another problem is when sharing code. While you may not be directly concerned yet, it is important to keep as much code as possible hidden from a potential user of your code. By putting your code into the header file, any programmer using your code must look the whole code, while there are just interested in how to use it. Putting your code in the cpp file allows to only deliver a binary component (a static or dynamic library) and its interface as header files, which may be simpler in some environment.

This is a problem if you want to be able to turn your current code into a dynamic library. Because you don't have a proper interface declaration decoupled from the actual code, you won't be able to deliver a compiled dynamic library and its usage interface as readable header files.

You may not have these issues yet, that's why I was telling that your solution may be ok in your current environment. But it is always better to be prepared to any change and some of these issues should be addressed.

PS: About C# or Java, you should keep in mind that these languages are not doing what you say. They are actually compiling files independently (like cpp files) and stores the interface globally for each file. These interfaces (and any other linked interfaces) are then used to link the whole project, that's why they are able to handle circular references. Because C++ does only one compilation pass per file, it is not able to globally store interfaces. That's why you are required to write them explicitely in header files.