Classes

[tomy12]

Hi guys,

The following code is a small part of a program I want to write for a question. The question says this: Create a class named B with a data member (e.g., a string) and another class named D with its own data member (say, another string) and that class D holds a value of class B.

I wrote the code below for it but face an error saying: identifier "_n" is undefined. But I have defined it in D(int _i, string _n).

What is the reason please? An how to solve it?

#include <iostream>
using namespace std;

class B {
public:
	B(string n) : name(n) {}
	string name;
};

//--------------------------

class D : public B {
public:
	D(int _i, string _n) : B(_n),i(_i) {}
	
	int i;
	B b1(_n);
};

//--------------------

int main() {
	return 0;
}

[2kaud]

   B b1(_n);

Why do you think that _n is defined here when it is only a parameter for the constructor?

[tomy12]

OK. I changed class D with this:

class D : public B {
public:
	D(int _i, string _n) : B(_n), i(_i), nd(_n) {}
	
	int i;
	string nd;
	B b1(nd);
};

But now get these:
Error 1 error C2061: syntax error : identifier 'nd' 18
2 IntelliSense: member "D::nd" is not a type name 18

[2kaud]

Correct! The error message should give a clue. If you saw the statement

B b1(int);

what would you think it was?

[tomy12]

I don't know what you mean really but B b1(int); is not correct because the class B gets an variable for its objects not a type. As far as it seems to me B b1(nd); is correct because nd is a variable string, what the class B needs to its objects.

[2kaud]

B b1(int);

look like a function declaration! So to the compiler

B b1(nd);

also looks like a function declaration with an unknown type for the parameter - hence the error message regarding unknown type !

Try

B b1 = B(nd);

Similarly, your original class D could be

class D : public B {
public:
	D(int _i, string _n) : B(_n), i(_i) {}

	int i;
	B b1 = B(name);
};

PS I don't know what compiler you are using, but with universal initialisation you could do

   B b1 {name};

or

   B b1 {nd};

[GCDEF]

FWIW, you assignment doesn't say anything about deriving D from B.

[tomy12]

Thank you 2Kuad.
B b1(nd); I don't know why it should look like a function declaration! nd is already defined.
B b1 = B(nd); seems odd to me. I have studied about class derivations but I don't recall that kind of making an object. Likely I've forgotten it. I think I should review that section of my book.

@GCDEF: Thanks for your advice. Would you please explain more?

And another question please:
If you had the question saying the following, what simplest code would you write for it? I used derivation method because I thought it's the only possible way (for having a value of a class as a member of another class) and it's also simple.
Question: Create a class named B with a data member (e.g., a string) and another class named D with its own data member (say, another string) and that class D holds a value of class B.

[GCDEF]

Thank you 2Kuad.
B b1(nd); I don't know why it should look like a function declaration! nd is already defined.
B b1 = B(nd); seems odd to me. I have studied about class derivations but I don't recall that kind of making an object. Likely I've forgotten it. I think I should review that section of my book.

@GCDEF: Thanks for your advice. Would you please explain more?

And another question please:
If you had the question saying the following, what simplest code would you write for it? I used derivation method because I thought it's the only possible way (for having a value of a class as a member of another class) and it's also simple.
Question: Create a class named B with a data member (e.g., a string) and another class named D with its own data member (say, another string) and that class D holds a value of class B.

There's not much more to say. You've derived D from B as in
class D : public B
which means D is derived from B. Your assignment doesn't say to do that. All you need is
class D