[SOLVED] Passing an enum value to a nested class constructor C++

just1alexx · 06-18-2011, 01:34 PM

Hi, the compiler (g++ 4.4.5) does not recognize PRE as a type in the following code:

Code:

enum mode {PRE, IN, POST};

class tree
{    
    public:   

        class iter
        {
            public:
            iter (mode m);
        };    
        
        tree (mode m); 

        iter alex (PRE);           /* PRE does not define a type */
};

Why?

lyle_s · 06-18-2011, 02:12 PM

Quote:

Originally Posted by just1alexx

Hi, the compiler (g++ 4.4.5) does not recognize PRE as a type in the following code:

Code:

enum mode {PRE, IN, POST};

class tree
{    
    public:   

        class iter
        {
            public:
            iter (mode m);
        };    
        
        tree (mode m); 

        iter alex (PRE);           /* PRE does not define a type */
};

Why?

Your code compiles:

Code:

lyle@bowman:~$ g++ --version
g++ (GCC) 4.4.4
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

lyle@bowman:~$ g++ -c test.cxx

but doesn't link:

Code:

lyle@bowman:~$ g++  test.o 
test.o: In function `main':
test.cxx:(.text+0x15): undefined reference to `tree::tree(mode)'
test.cxx:(.text+0x26): undefined reference to `tree::iter::iter(mode)'
collect2: ld returned 1 exit status

because you haven't provided definitions for 'tree::tree(mode)' and 'tree::iter::iter(mode)'; you have only provided declarations for these things.

In summary, the compiler does recognize 'PRE' as a value of type 'enum mode', so it does compile. It won't link because you haven't provided definitions/implementations of 'tree::tree(mode)' and 'tree::iter::iter(mode)'.

Lyle.

just1alexx · 06-18-2011, 02:20 PM

Not in mine:

Code:

alex@sabertooth: /mnt/data/Programming$ g++ --version
g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Code:

alex@sabertooth: /mnt/data/Programming$ g++ -c test.cpp
test.cpp:16: error: ‘PRE’ is not a type

alex@sabertooth: /mnt/data/Programming$

I know that I haven't provided any implementation yet, I was just compiling.

lyle_s · 06-18-2011, 02:43 PM

Sorry, I was wondering after typing that all up if I was answering your question, but I left it thinking you'd tell me if I wasn't.

I'm a little confused. Your code compiles at http://www.comeaucomputing.com/tryitout/ without error.

'PRE' isn't a type; it's a value of type 'mode', so g++ 4.4.5 is looking for a type there. Sorry, not sure what's going on.

Lyle.

just1alexx · 06-18-2011, 02:45 PM

No problem, I'll find a workaround, thanks for the help

dwhitney67 · 06-18-2011, 02:46 PM

Quote:

Originally Posted by just1alexx

Hi, the compiler (g++ 4.4.5) does not recognize PRE as a type in the following code:

Code:

...

        iter alex (PRE);           /* PRE does not define a type */
};

Why?

In the code above, I believe you are attempting to construct a variable named 'alex', with it's initial value set to PRE. However, to the compiler, what you are doing is declaring a function that accepts a PRE type, and returns an iter object. The data type PRE does not exist, and hence the error. Do *not* confuse data type with enum value.

Anyhow, the proper place to construct 'alex' is in the tree constructor.

Sorry for changing your variables names; I can't stand to see someone deviate from my coding standards.

Code:

enum Mode {PRE, IN, POST};

class Tree
{
public:
   class Iter
   {
   public:
      Iter(Mode mode);
   };

   Tree(Mode mode) : alex(PRE) {}

   Iter alex;
};

// missing implementation for Iter constructor.

int main()
{
   Tree tree(POST);
}

just1alexx · 06-18-2011, 02:56 PM

Ohhh... I see, I did not realize that it could be also interpreted as a prototype.
But then, wouldn't every public object that I try to declare be interpreted as a prototype also?

dwhitney67 · 06-18-2011, 03:08 PM

Quote:

Originally Posted by just1alexx

Ohhh... I see, I did not realize that it could be also interpreted as a prototype.
But then, wouldn't every public object that I try to declare be interpreted as a prototype also?

I'm not sure I understand what you are stating above; perhaps you could provide an example. Earlier you were attempting to declare and initialize a variable within the class, using a style that resembles that of a function declaration. You confused the compiler into believing that you were declaring a function. It did not know what data type PRE is. PRE is not a type; it is an enum value.

If you tried something like this:

Code:

Iter alex = PRE;

then the compiler will also complain. You cannot initialize variable within the class declaration, unless you declare it static and you are assigning a numeric (an int) value to the variable. PRE is not numeric; it is an enum value, and hence even the code above would not work with the static attribute.

just1alexx · 06-18-2011, 03:19 PM

nevermind, you just answered my question when edited your code. I see now, thanks a lot, now I know what to do.
Sorry for the prototype thing, is just another word that i use borrowed from C to say function declaration.

dwhitney67 · 06-18-2011, 03:24 PM

Yes, the correct usage of terms sometimes escapes me too.

function prototype = function declaration

function definition = function implementation