static map in a base class?

static map in a base class?

[Colin Law]

Hi,

Could somebody explain why the code below will compile, but causes a seg 
fault when run. It appears the the map is not initialized with the base 
class when creating a derived class from it... is this correct? Should 
the map be getting initialized with the base class or am I missing 
something here?

Suggestions anyone?

Regards,
~Colin.


//A.h
#ifndef __A_H__
#define __A_H__
#include <map>

class A
{
public:
  A() ;
  ~A() ;
  void doSomething() ;
  static std::map<int,std::string> theMap ;
};
#endif
-------------------------------------------------------
//B.h
#ifndef __B_H__
#define __B_H__
#include <map>
#include "A.h"

class B : public A
{
public:
  static B testB ;
  B() ;
  ~B() ;
  void doMore() ;
};
#endif
-----------------------------------------------------------
//A.cc
#include "A.h"
#include <iostream>
std::map<int,std::string> A::theMap ;

A::A()
{
  std::cout << "A :: Constructor" << std::endl ;
  std::cout << "Mapsize = " << theMap.size() << std::endl ;
  theMap.insert(std::pair<int,std::string>(1,"test")) ;
  std::cout << "Mapsize = " << theMap.size() << std::endl ;
}

A::~A()
{
  std::cout << "A :: Destructor" << std::endl ;
}

void
A::doSomething()
{
  std::cout << "Do Something" << std::endl ;
}
-----------------------------------------------------------
//B.cc
#include "B.h"
#include <iostream>
B B::testB ;

B::B()
{
  std::cout << "B :: Constructor" << std::endl ;
}

B::~B()
{
  std::cout << "B :: Destructor" << std::endl ;
}

void
B::doMore()
{
  std::cout << "Do More" << std::endl ;
}
-----------------------------------------------------------
//test.cc
#include "A.h"
#include "B.h"
#include <iostream>

int main()
{
  std::cout << "test" << std::endl ;

  //  B b ;
  return(0) ;
}

Re: static map in a base class?

[Eljay Love-Jensen]

Hi Colin,

A's static theMap will be initialized by the time main is called.

B's static testB will be initialized by the time main is called.  But note, 
it DEPENDS on A's static theMap being initialized BEFORE it gets 
initialized, and MAY BE initialized BEFORE A's static theMap is 
initialized!  There is NO guarantee of order-of-initialization between 
different compile modules.

One way to alleviate the cross-module initialization conundrum is via:
// In a.cpp
class A
{
public:
A() ;
~A() ;
void doSomething() ;
static std::map<int,std::string>& getMap();
};

// In a.h
std::map<int,std::string>& A::getMap() {
static std::map<int,std::string> A::theMap ;
return theMap;
}

The trick works because getMap's static theMap is GUARANTEED to be 
initialized the first time the routine is accessed.  Regardless of 
cross-module order-of-initialization dependencies.

- - - - - - - - - - - -

Some other nits:
You didn't #include <string>.
You didn't #include <utility>, for the pair template.
Your sentinels, e.g. __A_H__, are illegal.  I recommend a_h_ONCE instead.

Sincerely,
--Eljay

Re: static map in a base class?

[Colin Law]

Eljay Love-Jensen wrote:

> I may have had some typos in my pseudo-C++-code.  Nothing to hard to 
> touch up.
>
> :-)
>
> --Eljay
>
Eljay,

Thanks for the help, this has been bugging me for a while now.

~Colin.