There is something valueable for me,which took me much time off and brought much experience to me.

When we write constructor's implemention,we should pay attention to the order that we declare variables.

eg: class Box { private: int a; int b; public: Box(int n):b(n++),a(n++){} void show(){ cout << a << b << endl; } };

There, we write a class called Box.When we define a Box instantiate like this: Box k(3), how much do you think the value of a and b? The answer is that a equal to 3 and b equal to 4. although we write the initializtion like this:b(n++),a(n++),but the order initializing them is the order what we declare them.

When I use a header file to write template class,and the implementation in cpp,there is many errors like this:undefined reference to .... After that,I look up about much time,but no satisfied answer(which says not including .h to the main cpp). Last,stackoverflow gives me a perfect answer,so I want to share the URL:http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file

and the reason is that although we write implementation in cpp,the code won't be created by compiling that cpp file.

eg:there are some files. foo.h declares the interface of class MyClass declares the interface of class Box foo.cpp defines the implementation of class MyClass defines the implementation of class Box bar.cpp uses MyClass uses Box

When foo.cpp is compiled,the corresponding code won't be generated.So when compiler compiles bar.cpp and wants to create a MyClass,it can't see MyClass(can only see the interface in foo.h) and create Myclass.However,the Box's code can be created.Because when compiling foo.cpp,the code of Box has already been created.