Is is a good practice to put the definition of C++ classes into the header file?

Question

When we design classes in Java, Vala, or C# we put the definition and declaration in the same source file. But in C++ it is traditionally preferred to separate the definitio

Answer 1

Two particular problems with putting everything in the header:

1. Compile times will be increased, sometimes greatly. C++ compile times are long enough that that's not something you want.

2. If you have circular dependencies in the implementation, keeping everything in headers is difficult to impossible. eg:

   header1.h

   struct C1
   {
     void f();
     void g();
   };
   

   header2.h

   struct C2
   {
     void f();
     void g();
   };
   

   impl1.cpp

   #include "header1.h"
   #include "header2.h"
   
   void C1::f()
   {
     C2 c2;
     c2.f();
   }
   

   impl2.cpp

   #include "header2.h"
   #include "header1.h"
   
   void C2::g()
   {
     C1 c1;
     c1.g();
   }