Help with undefined references

[sieg1974]

Hi,

I know it must be a simple mistake, but I'm just learning C++ and can't
figure out what's wrong. Any help will be vey much appreciated
I got the following error messages when the following program is
compiled and linked. What could be wrong?

Thanks,

Andre

/tmp/ccX0Z0Il.o(.text+0x17d): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
/tmp/ccX0Z0Il.o(.text+0x1ca): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
/tmp/ccX0Z0Il.o(.text+0x1cf): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
collect2: ld returned 1 exit status

#include <iostream>

class Singleton
{
public:
static Singleton * Instance();
~Singleton();

private:
Singleton();
static Singleton * theSingleton;
};

Singleton::Singleton()
{
std::cout << "Singleton constructot\n";
}

Singleton::~Singleton()
{
std::cout << "Singleton destructor\n";
}

Singleton * Singleton::Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();
}

return( theSingleton );
}

Singleton * theSingleton = 0;

int main()
{
return( 0 );
}

 

[Greg]

Hi,

I know it must be a simple mistake, but I'm just learning C++ and can't
figure out what's wrong. Any help will be vey much appreciated
I got the following error messages when the following program is
compiled and linked. What could be wrong?

Thanks,

Andre

/tmp/ccX0Z0Il.o(.text+0x17d): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
/tmp/ccX0Z0Il.o(.text+0x1ca): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
/tmp/ccX0Z0Il.o(.text+0x1cf): In function `Singleton::Instance()':
: undefined reference to `Singleton::theSingleton'
collect2: ld returned 1 exit status

#include <iostream>

class Singleton
{
public:
static Singleton * Instance();
~Singleton();

private:
Singleton();
static Singleton * theSingleton;
};

Singleton * theSingleton = 0;

Should be:

Singleton * Singleton::theSingleton = 0;

The member name needs to be qualified with its class name:

Greg

 

[Isold.Wang]

You can see this book <<Modern C++ Design>>
Chapter 6. Implementing Singletons

 

[sieg1974]

Thanks for the help, I could make it work now. But I have another
question .
I have the following 2 versions of my program. The first one works
fine, but not the second one. I get the same linking erros as before.
Can anyone point out what could be wrong?

Thanks,

Andre

***********
Version 1
***********

#include <iostream>

class Singleton
{
public:
static Singleton * Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();
}

return( theSingleton );
}

private:
static Singleton * theSingleton;

};

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );
}

***********
Version 2
***********

#include <iostream>

class Singleton
{
public:
static Singleton * Instance();

private:
static Singleton * theSingleton;

};

Singleton * Singleton::Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();
}

return( theSingleton );
}

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );
}

 

[Victor Bazarov]

Thanks for the help, I could make it work now. But I have another
question .
I have the following 2 versions of my program. The first one works
fine, but not the second one. I get the same linking erros as before.

Same, which?

Can anyone point out what could be wrong?

Thanks,

Andre

***********
Version 1
***********

#include <iostream>

^^^^^^^^^^^^^^^^^^^
You don't need this since you use nothing from that header.

class Singleton
{
public:
static Singleton * Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();
}

return( theSingleton );

I really wish you'd drop the parentheses around the expression in the
'return' statement. Makes your code look amateurish. 'return' is not
a function. There is no need for parentheses!

}

private:
static Singleton * theSingleton;

};

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );
Ugh!

}

***********
Version 2
***********

#include <iostream>

class Singleton
{
public:
static Singleton * Instance();

private:
static Singleton * theSingleton;

};

Singleton * Singleton::Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();
}

return( theSingleton );
Yuck!

}

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );
Yechh!

}

I don't get any link errors if I try building your programs from
the source code you posted.

V

 

[Salt_Peter]

Thanks for the help, I could make it work now. But I have another
question .
I have the following 2 versions of my program. The first one works
fine, but not the second one. I get the same linking erros as before.
Can anyone point out what could be wrong?

Thanks,

Andre

***********
Version 1
***********

#include <iostream>

class Singleton
{
public:
static Singleton * Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();

try:
theSingleton = new Singleton;

}

return( theSingleton );

return theSingleton;

}

private:
static Singleton * theSingleton;

};

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );
}

***********
Version 2
***********

#include <iostream>

class Singleton
{
public:
static Singleton * Instance();

private:
static Singleton * theSingleton;

};

Singleton * Singleton::Instance()
{
if( !theSingleton )
{
theSingleton = new Singleton();

same changes...
theSingleton = new Singleton;

}

return( theSingleton );
}

Singleton * Singleton::theSingleton = 0;

int main()
{
return( 0 );

return 0;