Linking problem in VC++

Posted on 1998-09-01
Last Modified: 2010-04-02
   I have the following class:

   #include <iostream.h>
   #include <string.h>
   #include <stdio.h>
   #ifndef STR
   #define STR
   class TPString
   char *buf; // to contain the data
   int len; // length of this buffer
        // Constructors
   TPString(  char *aBuf );
   TPString(  TPString& rhs );
   // Destructors
           // Concatenations
     TPString  operator+(  TPString& rhs );
   friend TPString  operator+ (  char* lhs,  TPString& rhs );
            // Assignments
   TPString& operator=(  TPString& rhs );
   TPString& operator=(  char* rhs );
   When I compile and build my main function, I have the following link errors:

   main.obj : error LNK2001: unresolved external symbol "public: __thiscall TPString::~TPString(void)"
   main.obj : error LNK2001: unresolved external symbol "public: __thiscall TPString::TPString(class TPString &)"
   main.obj : error LNK2001: unresolved external symbol "public: class TPString & __thiscall
   TPString::operator=(char *)" (??4TPString@@QAEAAV0@PAD@Z)
   main.obj : error LNK2001: unresolved external symbol "public: __thiscall TPString::TPString(void)"
   main.obj : error LNK2001: unresolved external symbol "class ostream & __cdecl operator<<(class ostream
   &,class TPString &)" (??6@YAAAVostream@@AAV0@AAVTPString@@@Z)
   main.obj : error LNK2001: unresolved external symbol "public: class TPString & __thiscall
   TPString::operator=(class TPString &)" (??4TPString@@QAEAAV0@AAV0@@Z)

   Can any expert please advise what happens and how can i solve/prevent the same errors.

   Thanks very much.
Question by:tanmh

Accepted Solution

Answers2000 earned 80 total points
ID: 1171806
The class definition above defines the structure of the class, however you have not defined the actual code that implements the class anywhere.  This is why.  When you call one of the functions in the class, there is no code anywhere to actually implement the functon.

In case you are wondering the reason the names are listed strangly is because the C++ compiler "mangles" this names with extra characters indicate the type of parameters that the function takes.  For example you have multiple constructors (each of the 3 functions called TPString after public) however each takes different parameters, so they are really different functions.  The linker needs a unique name for each unique function, so the C++ compiler adds the extra characters to the names to make these names unique.

1. You need to define the implementation code for each function that you have in the class,  

i) you can either do this in the body of the class
example (please note the absence of a semi-colon (;) after the function definition)

class TPString
        // Constructors
     // <--- add your code here



ii) add the code in a separate function body outside the { } enclosing the class


   class TPString
   } ;

TPString::TPString(  char *aBuf )
  // whatever code you want goes here
The line beginning "friend" in the class definition defines a global function (called operator+, which can be called by writing a + with the parameters stated) defines a global function which has access to the private data in a TPString.  This global function should also be defined somewhere, something like:

TPString  operator+ (  char* lhs,  TPString& rhs )
  TPString str(lhs) ;
  TPString str2 = str + rhs ;
  return str2 ;


Author Comment

ID: 1171807
Thanks very much for the clear and concise answer.
I've done the recommended ammendments and it works!!!!
Thanks a million.

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