1

I've searched through the web an didn't find any explanation why the following happens.

For example, there is a template class Enclosing with a nested class Nested.

In Enclosing class, there is a method that should create an instance of the Nested class and use it's fields and methods.

In the following code, there is a model of how I'm trying to do it: 

template<typename T, typename S>
class Enclosing{
public:
    class Nested;
    Nested foo();
};

template<typename T, typename S>
class Enclosing<T,S>::Nested{
public:
    T field;
    void some_method();
    friend class Enclosing;    // instead of this line I also tried:
                               // friend class Enclosing<T,S> 
                               // and it didn't work either
};

template<typename T, typename S>
typename Enclosing<T,S>::Nested Enclosing<T,S>::foo (){
    Nested nes;
    nes.some_method;         // the problem appears here
    return enc;
}

The problem is:

When I'm writing nes.some_method, none of the environments I tried (VS2010, eclipse), after I type "nes.", doesn't propose me any options. I seems like "nes" is not an instanse of the class at all.

How to access methods and fields of the nested class from enclosing template class?

Grinch
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2 Answers2

5

This line:

Nested nes();

Does not create an object of type nes, but rather declares a function which takes no arguments and returns an object of type Nested. I suspect this to be the source of your problem, not the friend declaration. Just remove the pair of parentheses after nes:

Nested nes;

Alternatively, in C++11 you may do:

Nested nes{};

EDIT:

After fixing the above error, it still seems your program does not compile & link correctly - doubtfully because of the same problem. What I can tell from your code is that a definition of the some_method() member function is still missing, and that may be a reason for the linker to refuse creating the executable for your program.

Andy Prowl
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  • I would really like to know what it is that leads people to making this error so frequently. I honestly don't ever remember doing this, yet somebody posts about it almost every day. – Ryan Witmer May 10 '13 at 15:42
  • @RyanWitmer The default constructor of a class takes no arguments, so beginners commonly get the misconception that they're calling the default constructor when instantiating an object. The syntax to it is also similar to calling a specialized constructor, which adds to the confusion. – David G May 10 '13 at 15:44
  • I just tried to remove parentheses and it still doesn't work. – Grinch May 10 '13 at 15:57
  • @Grinch: What do you mean by "it doesn't work"? There's 101 ways something cannot work, I cannot guess which one it is ;) Does it not compile? Does it not link? Does it crash? If it is a compiler error or linker error, what is the exact error message that you are seeing? – Andy Prowl May 10 '13 at 15:59
  • @Grinch: For instance, [this compiles and runs](http://ideone.com/dsmG8R). Notice, that I had to add a definition for `some_method()`, which was not shown in your original code. – Andy Prowl May 10 '13 at 16:00
  • @AndyProwl : Well, your example shows me that there is no problem :) at least, in the place I thought it is. – Grinch May 10 '13 at 16:13
  • @Grinch: OK, glad I could help at least in part ;) – Andy Prowl May 10 '13 at 16:19
0

Here:

Nested nes();

You're not calling the default constructor of Nested but rather declaring a function named nes that takes 0 arguments and returns by value an instance of the Nested class. This is referred to as the Most Vexing Parse. Remove the parameters to get the code to function properly.

Here is a working demo.

David G
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