What gives such a design?

extern class Class obj {};

enter image description here which is not correct in the code in the pictures. enter image description here

  • Where did you meet this record? - αλεχολυτ

2 answers 2

This is an erroneous record that the compiler should have warned you about. extern (in such a key) is applied only to variables and functions, and you have neither one nor the other. If we modify the example a little:

 extern class Class { } instance;

, it turns out that we declared an instance variable of type Class .

The code above is just an abbreviated form of such a record:

 class Class { }; extern Class instance;

Those. we defined a new class Class and declared an object of type Class named instance . Somewhere we have to define this object, otherwise when using it we will get an error at the layout stage.

  • one
    why not just write extern Class instance; - Stanislav Petrov
  • @StanislavPetrov, because the syntax of the language does not allow this. What is the Class in this entry? - ixSci
  • your code says that somewhere there is a declaration of type Class and declares an object of this class. With such an object declaration, is it not necessary to connect a header with a real class declaration? - Stanislav Petrov
  • @StanislavPetrov, updated the answer. - ixSci

To the question why the word extern generally needed:

Suppose we have some global variable int var , this variable is one for the whole program (let's leave the question about the undesirability of global variables behind the scenes, the example is synthetic). A project consists of several .cpp files, each of which needs access to this variable. How to implement?

Option 1

 // header.h void foo(); int var; 

 // foo.cpp #include "header.h" int var = 42; void foo() { var = 42; } 

 // main.cpp #include <iostream> #include "header.h" int main() { foo(); std::cout << var; }

But no, when compiling we get this:

 foo.cpp:4:5: error: redefinition of 'int var' int var; ^~~ In file included from foo.cpp:2:0: header.h:5:5: note: 'int var' previously declared here int var; ^~~

This is all because we need to declare a variable in the header file, and we define it.

In header.h next to the variable, we declared the function foo:

 void foo();

In fact, we told the compiler - "This is what the foo function looks like, but its definition will be later (possibly in another file)." How to say the same about variables?

We need a tool for declaring variables.

That is the extern . Change one line in header.h :

 // header.h void foo(); extern int var;

And everything works:

 $ g++ main.cpp foo.cpp $ a.exe 42

extern works with functions, in fact these two declarations are equivalent:

 extern void foo(); // так обычно не пишут, но это верно void foo();

Starting with C ++ 11, there is another use case.

extern works only with functions and variables, but not with classes / structures, your construction does not make sense (and will not compile). If you need to declare a class, you need to write like this

 class Foo; struct Bar;

PS Another extern is used for a bunch of C ++ with C (and other languages), but that's another story ...