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default variable as a pointer in class

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Join Date: Apr 2008

Posts: 3

Reputation: Gondt is an unknown quantity at this point

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Gondt

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Newbie Poster

default variable as a pointer in class

Apr 20th, 2008

the title describes the problem. here's how the code looks like:

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#include <cstdio>
 
class example
{
	public:
	class node
	{
		public:
		int value;
		node *left, *right;
	};
 
	node *root;
 
	example (int a)
	{
		root=new node;
		root->value=a;
	}
 
// HERE'S THE PROBLEM:
	void function (node *variable=root)
	{
 
	}
};
 
int main()
{
 
}
#include <cstdio>

class example
{
	public:
	class node
	{
		public:
		int value;
		node *left, *right;
	};

	node *root;

	example (int a)
	{
		root=new node;
		root->value=a;
	}

// HERE'S THE PROBLEM:
	void function (node *variable=root)
	{

	}
};

int main()
{

}

g++ returns:

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p.cpp:13: error: invalid use of non-static data member 'przyklad::root'
p.cpp:21: error: from this location
p.cpp:13: error: invalid use of non-static data member 'przyklad::root'
p.cpp:21: error: from this location

i don't want to use static... do you know what is the proper way of coding it?

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Join Date: Aug 2005

Posts: 15,632

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Ancient Dragon Ancient Dragon is offline

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Still Learning

Re: default variable as a pointer in class

Apr 20th, 2008

Here is an explaination of default parameter values. That link talks about exactly what you are trying to do.

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.

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Join Date: Apr 2008

Posts: 3

Reputation: Gondt is an unknown quantity at this point

Solved Threads: 0

Gondt

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Newbie Poster

Re: default variable as a pointer in class

Apr 20th, 2008

i cannot find the answer there. i have no problem in using default parameters, except this particular case.

may it be somewhat connected to the fact, that if i put there

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void function (node *variable=this->root)
void function (node *variable=this->root)

i get

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p.cpp:21: error: 'this' may not be used in this context
p.cpp:21: error: 'this' may not be used in this context

a correct code would be highly appreciated

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Join Date: Aug 2005

Posts: 15,632

Reputation: Ancient Dragon has a reputation beyond repute

Solved Threads: 1497

Ancient Dragon Ancient Dragon is offline

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Still Learning

Re: default variable as a pointer in class

Apr 20th, 2008

>> cannot find the answer there.
Click on the link "Restrictions on Default Arguments" in the left pannel.

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You cannot use local variables in default argument expressions. For example, the compiler generates errors for both function g() and function h() below:
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void f(int a)
{
      int b=4;
      void g(int c=a); // Local variable "a" cannot be used here
      void h(int d=b); // Local variable "b" cannot be used here
}
void f(int a) { int b=4; void g(int c=a); // Local variable "a" cannot be used here void h(int d=b); // Local variable "b" cannot be used here }