compiling a source file which contains a function declared in a header

Question

Suppose there is a source file un2.cpp

----un2.cpp----

class employee;
void employee::setname(string s)
{
.....(some  code)
}

The employee class is defined in un2.h wherein setname() is declared. Now when i attempt to compile un2.cpp by 'cc -c un2.cpp -o un2.o', i get an error message

 un2.cpp:3:16: error: invalid use of incomplete type ‘struct employee’
 un2.cpp:1:7: error: forward declaration of ‘struct employee’

Why doesn't the compiler just take employee as an external symbol(like it would take a previously declared but undefined symbol) and compile the un2.cpp file.

Is there any difference between compiling a normal undefined symbol and a undefined class? Also can anyone tell me(if it is possible), the way to just forward declare a class and define the symbols in it?

No...if i included the .h file the preprocessor would copy it in .cpp file and there would be no external symbol to talk about. It would be like a normal compilation with everything defined. — AvinashK
– AvinashK, Commented Aug 15, 2012 at 14:33

AndersK · Accepted Answer · 2012-08-15 13:07:35Z

3

external is for the linker and will not help you when you compile your code since the compiler does not know your class declaration by just writing a forward declaration.

Instead you need to include the header with the declaration in your un2.cpp file in order to compile.

answered Aug 15, 2012 at 13:07

AndersK

36.1k6 gold badges64 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Lyubomir Vasilev · Accepted Answer · 2012-08-15 13:10:54Z

2

You have not included the file with the declaration of the class. Forward declaration does not help here, because the compiler doesn't know whether the class contains such a method.

Include the header file and the error should go away.

answered Aug 15, 2012 at 13:10

Lyubomir Vasilev

3,04020 silver badges25 bronze badges

Comments

celtschk · Accepted Answer · 2012-08-15 13:13:16Z

The point is that a class is not represented by a symbol, but the complete class structure must be known for code generation. To start with, the compiler needs the class definition to check that the class employee indeed contains a member function named setname taking a string and returning void (if it doesn't, the compiler must return an error). Moreover, you'll want to access other members of the class inside the function (call other member functions, or access data members), and therefore you'll need the declarations of those, which are part of the class definition. At linkage time, the class name is completely gone (except for debugging information, and as part of the symbols of member functions and for the virtual table).

Generally when writing an implementation file for a header, you should include the header file min the implementation file. This will make sure that you have everything needed to define everything declared there, and besides that also tests the header file, provided you include it as very first thing in the implementation file.

Collectives™ on Stack Overflow

compiling a source file which contains a function declared in a header

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related