Build errors on "trivial" proof of concept design pattern

[Alfonso Morra]

Hi,

I've written a very simple Simpleton design pattern class template. This
was supposed to illustrate a point. Although it compiles without nay
errors, I get link errors (unresolved external), during linking - before
anyone jumps to conclusions that this is OT, please check the linker
error message below, it is NOT OT.

Here are my class declarations/definitions:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;
Singleton(const Singleton&) {}
void operator=(const Singleton&) {}
};
template <class T>
T& Singleton<T>::i nstance() {
if (!instance_)
instance_ = new T ;

return *instance_;
}
//Simple Test harness
class Test : public Singleton<Test> {
friend class Singleton<Test> ;

public:
int foo(void){ return 1 ; }
void bar(void) { ; }
virtual ~Test() { cout << "Dtor called" << endl ; }

private:
Test() { cout << "Ctor called" << endl ; }
};

The code to test this is below (header stuff not included for brevity)

int main() {
Foo &f = Foo::instance() ;

}
Here is the linker eror I get:

singleton error LNK2001: unresolved external symbol "private: static
class Test * Singleton<class Test>::instance _"

Can anyone spot where I'm going wrong?

[BigBrian]

> Hi,

I've written a very simple Simpleton design pattern class template. This
was supposed to illustrate a point. Although it compiles without nay
errors, I get link errors (unresolved external), during linking - before
anyone jumps to conclusions that this is OT, please check the linker
error message below, it is NOT OT.

Here are my class declarations/definitions:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;
Where is this static member instanciated?
Singleton(const Singleton&) {}
void operator=(const Singleton&) {}
};
template <class T>
T& Singleton<T>::i nstance() {
if (!instance_)
instance_ = new T ;

return *instance_;
}
//Simple Test harness
class Test : public Singleton<Test> {
friend class Singleton<Test> ;

public:
int foo(void){ return 1 ; }
void bar(void) { ; }
virtual ~Test() { cout << "Dtor called" << endl ; }

private:
Test() { cout << "Ctor called" << endl ; }
};

The code to test this is below (header stuff not included for brevity)

int main() {
Foo &f = Foo::instance() ;

}
Here is the linker eror I get:

singleton error LNK2001: unresolved external symbol "private: static
class Test * Singleton<class Test>::instance _"

Can anyone spot where I'm going wrong?

I don't see where Singleton<Test> ::instance_ is being instanciated.

-Brian

[Alfonso Morra]

BigBrian wrote:

Hi,

I've written a very simple Simpleton design pattern class template. This
was supposed to illustrate a point. Although it compiles without nay
errors, I get link errors (unresolved external), during linking - before
anyone jumps to conclusions that this is OT, please check the linker
error message below, it is NOT OT.

Here are my class declarations/definitions:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;

Where is this static member instanciated?

Singleton(const Singleton&) {}
void operator=(const Singleton&) {}
};
template <class T>
T& Singleton<T>::i nstance() {
if (!instance_)
instance_ = new T ;

return *instance_;
}
//Simple Test harness
class Test : public Singleton<Test> {
friend class Singleton<Test> ;

public:
int foo(void){ return 1 ; }
void bar(void) { ; }
virtual ~Test() { cout << "Dtor called" << endl ; }

private:
Test() { cout << "Ctor called" << endl ; }
};

The code to test this is below (header stuff not included for brevity)

int main() {
Foo &f = Foo::instance() ;

}
Here is the linker eror I get:

singleton error LNK2001: unresolved external symbol "private: static
class Test * Singleton<class Test>::instance _"

Can anyone spot where I'm going wrong?

I don't see where Singleton<Test> ::instance_ is being instanciated.

-Brian

Its been done in the instance method (T& Singleton<T>::i nstance())

[Karl Heinz Buchegger]

Alfonso Morra wrote:

Its been done in the instance method (T& Singleton<T>::i nstance())

No.
What BigBrian ment is:
You *declared* a variable instance_, here:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;

This says: somewhere there is a variable called instance_. It is
a member of template <class T> Singleton.
But you never told the compiler to actually *create* that variable.
It is like: you have a function prototype, which tells everybody:
Somwhere there is a function called .... But then you actually
need to implement it!

eg.

class X
{
// This *declares* the member variable
static int x;
};

int X::x = 5; // and this *defines* (implements) the variable.

--
Karl Heinz Buchegger
kb******@gascad .at

[BigBrian]

> BigBrian wrote:

Hi,

I've written a very simple Simpleton design pattern class template. This
was supposed to illustrate a point. Although it compiles without nay
errors, I get link errors (unresolved external), during linking - before
anyone jumps to conclusions that this is OT, please check the linker
error message below, it is NOT OT.

Here are my class declarations/definitions:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;

Where is this static member instanciated?

Singleton(const Singleton&) {}
void operator=(const Singleton&) {}
};
template <class T>
T& Singleton<T>::i nstance() {
if (!instance_)
instance_ = new T ;

return *instance_;
}
//Simple Test harness
class Test : public Singleton<Test> {
friend class Singleton<Test> ;

public:
int foo(void){ return 1 ; }
void bar(void) { ; }
virtual ~Test() { cout << "Dtor called" << endl ; }

private:
Test() { cout << "Ctor called" << endl ; }
};

The code to test this is below (header stuff not included for brevity)

int main() {
Foo &f = Foo::instance() ;

}
Here is the linker eror I get:

singleton error LNK2001: unresolved external symbol "private: static
class Test * Singleton<class Test>::instance _"

Can anyone spot where I'm going wrong?

I don't see where Singleton<Test> ::instance_ is being instanciated.

-Brian

Its been done in the instance method (T& Singleton<T>::i nstance())

But Singleton<Test> ::instance_ is static, so it needs to be
instanciated at file scope.

-Brian

[Alfonso Morra]

Alfonso Morra wrote:

BigBrian wrote:

Hi,

I've written a very simple Simpleton design pattern class template. This
was supposed to illustrate a point. Although it compiles without nay
errors, I get link errors (unresolved external), during linking - before
anyone jumps to conclusions that this is OT, please check the linker
error message below, it is NOT OT.

Here are my class declarations/definitions:

template <class T>
class Singleton {

public:
static T& instance();
protected:
Singleton() {}

private:
static T* instance_ ;

Where is this static member instanciated?

Singleton(const Singleton&) {}
void operator=(const Singleton&) {}
};
template <class T>
T& Singleton<T>::i nstance() {
if (!instance_)
instance_ = new T ;

return *instance_;
}
//Simple Test harness
class Test : public Singleton<Test> {
friend class Singleton<Test> ;

public:
int foo(void){ return 1 ; }
void bar(void) { ; }
virtual ~Test() { cout << "Dtor called" << endl ; }

private:
Test() { cout << "Ctor called" << endl ; }
};

The code to test this is below (header stuff not included for brevity)

int main() {
Foo &f = Foo::instance() ;

}
Here is the linker eror I get:

singleton error LNK2001: unresolved external symbol "private: static
class Test * Singleton<class Test>::instance _"

Can anyone spot where I'm going wrong?

I don't see where Singleton<Test> ::instance_ is being instanciated.

-Brian

Its been done in the instance method (T& Singleton<T>::i nstance())

My bad. Thanks for the correction guys. The ff file scope definition
fixed it:

template <class T>
T * Singleton<T>::i nstance_ = NULL ;