CS210 Lab: Templates--Hint for Using with Visual C++

Overview:

• If you try to define templates in multiple files, it is likely that you will come across a linker error in Visual C++. The error might be something like the following:

  main.obj : error LNK2001: unresolved external symbol "public: __thiscall LinkedList::~LinkedList(void)" (??1?$LinkedList@H@@QAE@XZ)
  

• To get rid of this error, you need to tell the Visual C++ compiler in what order to compile the files and only to compile them once.
• You do this by using some preprocessor directives, which are instructions to the compiler.
• All preprocessor directives begin with a # sign.
• You have already been using one preprocessing directive: #include.
• To get rid of the linker error when using templates, you can use three new preprocessor directives:
  • #ifndef
  • #endif
  • #define
• The general form of #ifndef is:

  #ifndef name
  	statement sequence
  #endif
  

• This means that if name is currently undefined, then compile all the code up to the matching #endif line.
• Using the following format in a file, the code up to the #endif will only be compiled once. If the compiler encounters the file again during the compilation, name will be already defined and all code up to and including the #endif will be ignored.

  #ifndef name
  #define name
          statement sequence
  #endif
  

• Note: name might be something like "LINKEDLIST_H". The tradition for identifier names is that they should match the file name. The period character ('.') is not valid for identifiers, so it is replaced by the underscore ('_') character.

• Putting it all together, the general format for all the files should be as follows:

  for each .h file, add-in
  
  #ifndef filename_h
  #define filename_h
  	statement sequence
  
  #endif
  #include "filename.cpp"
  
  for each .cpp file, add in
  
  #ifndef filename_cpp
  #define filename_cpp
  #include "filename.h"
  
           statment sequence
  
  #endif
  
  in main 
  #include "filename.h"
  	statement sequence
  

• Again, this tells the Visual C++ compiler in what order to compile the files and compiles them only once.