C / C++ / MFC

Shy Agam wrote:

It seems that this is some kind of linkage problem of some sort, but I can't figure it out.

#include <HeaderFileFromLibrary.h>

using namespace NamespaceFromHeaderFile;

int main(int argc, char **args)
{
    ClassFromHeaderFile *pClass = ClassFromHeaderFile::GetInstance();
    
    .
    .
    .

    return 0;
}

Cedric Moonen wrote:

Are you sure you linked correctly to the library ?

Shy Agam wrote:

As far as I can tell I did link the library correctly.

paul_71 wrote:

No, what he's getting is an linker error.

paul_71 wrote:

these hierarchical"replies" suck!

Richard MacCutchan wrote:

Is GetInstance() a static member with public visibility?

Aescleal wrote:

PS: It's also identical to the behaviour of global variables - declare in a header, define in source. Which is a big hint why statics are bad, bad, bad...

// A.h

class A
{
    private:
        static B b;
};

// A.cpp

A::B b( b_init );

// A.cpp

namespace
{
    B b;
}

Aescleal wrote:

So where would you use a class static instead of just declaring a variable in an anonymous namespace?

Aescleal wrote:

They've got exactly the same visibility, thread safety issues and initialisation order problems

Tim Craig wrote:

I suppose the classic example is where you want to keep stats on the class such has how many instances are currently running rampant.

template<class T> 
class InstanceCountable { :) 
 static int sCounter;
protected:
 InstanceCountable() {
  InstanceCountable<T>::sCounter++;
 }
 ~InstanceCountable() {
  InstanceCountable<T>::sCounter--;
 }
public:
 static GetCount() {
  return InstanceCountable<T>::sCounter;
 }
};

template<class T>
int InstanceCountable<T>::sCounter(0);

// ...and then

class A : public InstanceCountable<A> {
 //...
};

class B : public InstanceCountable<B> {
 //...
};

// etc...

int main() {
 A a1, a2;
 std::array<B, 10> barr;

 std::cout << 
  A::GetCount() << std::endl // prints 2
  B::GetCount() << std::endl;// prints 10
}

// in AStuff.h
class AStuff {
public:
 void Hello() {
  // ...
 }
};

// in A.h
class A {
public:
 static AStuff sStaticAMember; // <-declaration
};

// in A.cpp
AStuff A::sStaticAMember; //<-definition (required!)