Multiple includes
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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...
That's a rather sweeping statement. Parts of the language are there for a reason. There are cases where you absolutely need them. Can you misuse them? Certainly. But just yelling bad, bad, bad, ignores the valid cases.
Once you agree to clans, tribes, governments...you've opted for socialism. The rest is just details.
So where would you use a class static instead of just declaring a variable in an anonymous namespace? i.e. where would you use:
// A.h
class A
{
private:
static B b;
};// A.cpp
A::B b( b_init );
over:
// A.cpp
namespace
{
B b;
}'Cause I'm not sure I can see any case where that would be of any use. And if you're talking about using public class static data member why not use a global? They've got exactly the same visibility, thread safety issues and initialisation order problems.
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So where would you use a class static instead of just declaring a variable in an anonymous namespace? i.e. where would you use:
// A.h
class A
{
private:
static B b;
};// A.cpp
A::B b( b_init );
over:
// A.cpp
namespace
{
B b;
}'Cause I'm not sure I can see any case where that would be of any use. And if you're talking about using public class static data member why not use a global? They've got exactly the same visibility, thread safety issues and initialisation order problems.
Aescleal wrote:
So where would you use a class static instead of just declaring a variable in an anonymous namespace?
...Whenever the non-local is associated with a type, think of the "class object" idioms: reflectors, serializers, registrars etc... Especially in generic code, class statics can be looked up parametrically, which can't be achieved with traditional non-locals.
Aescleal wrote:
They've got exactly the same visibility, thread safety issues and initialisation order problems
This remains true and makes c++ non-locals of either kind a dangerous business...
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So where would you use a class static instead of just declaring a variable in an anonymous namespace? i.e. where would you use:
// A.h
class A
{
private:
static B b;
};// A.cpp
A::B b( b_init );
over:
// A.cpp
namespace
{
B b;
}'Cause I'm not sure I can see any case where that would be of any use. And if you're talking about using public class static data member why not use a global? They've got exactly the same visibility, thread safety issues and initialisation order problems.
I suppose the classic example is where you want to keep stats on the class such has how many instances are currently running rampant. Why should I have to go reference another namespace? Like many things in the C world, there's plenty there to shoot yourself in the foot with, you need to exercise proper caution.
Once you agree to clans, tribes, governments...you've opted for socialism. The rest is just details.
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No! You may declare your static class members only once. What you have missed, is the definition (which also has to be unique), and it needs to be provided outside of the class, for non-const and non-trivial objects.
// 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!)The constructor for
A::sStaticAMemberwill be called by the runtime (dynamic initialization) if the instance is used by the program. You do not (really) have the control over when this is going to happen, neither when the destructor is going to be called. This is one of the reason why the poster above damned "statics" ... and basically I have to agree with him. Are you using C# otherwise? If yes, then forget about the comfort of static class objects and their well defined initialization there - and enter one of the many dark zones of c++. I assume you will probably run into the problem of undefined order of initializations of your non-local objects, but we stand ready to help :)modified on Tuesday, August 31, 2010 12:56 PM
This is all a bit overwhelming for me... I also read some articles regarding the subject, but I still can't <b><i>completely</i></b> get my head around it. I should hit the books for a while and establish a more solid understanding of C++'s principles and design issues. For now, I would add this question though... What is the "danger" in what I did? i.e. declaring a private static member variable inside a class...
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I suppose the classic example is where you want to keep stats on the class such has how many instances are currently running rampant. Why should I have to go reference another namespace? Like many things in the C world, there's plenty there to shoot yourself in the foot with, you need to exercise proper caution.
Once you agree to clans, tribes, governments...you've opted for socialism. The rest is just details.
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.
Yes, this is a classic one, where a static member must be associated with a class... As an example to the generic argument from above, consider an
InstanceCountabepolicy: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
} -
This is all a bit overwhelming for me... I also read some articles regarding the subject, but I still can't <b><i>completely</i></b> get my head around it. I should hit the books for a while and establish a more solid understanding of C++'s principles and design issues. For now, I would add this question though... What is the "danger" in what I did? i.e. declaring a private static member variable inside a class...
Well, there are several "dangers" in that, but don't panic! It's a perfectly legal c++ construct - which can be used in safe manner if: 1. the functionality of your program does not depend on the time when the global objects are initialized/deinitialized 2. the globals do not depend on the state of each other in an undefined manner 3. you can assure that these global objects are initialized at all - speaking in terms of the holy standard "they are used in the program". With regards to point 3: Under certain circumstances, linkers do not include these globals into the program, if they don't see an explicit usage of them. This behavior collides with many popular idioms such as pluggable factories, reflectors or serializers: Here, the framework relies on dynamic initialization (this is a process which runs before your program starts) of static class objects with non-trivial constructors which perform certain functionality on instantiation - e.g. register a factory method for a type in an another global collection of factory methods.
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Well, there are several "dangers" in that, but don't panic! It's a perfectly legal c++ construct - which can be used in safe manner if: 1. the functionality of your program does not depend on the time when the global objects are initialized/deinitialized 2. the globals do not depend on the state of each other in an undefined manner 3. you can assure that these global objects are initialized at all - speaking in terms of the holy standard "they are used in the program". With regards to point 3: Under certain circumstances, linkers do not include these globals into the program, if they don't see an explicit usage of them. This behavior collides with many popular idioms such as pluggable factories, reflectors or serializers: Here, the framework relies on dynamic initialization (this is a process which runs before your program starts) of static class objects with non-trivial constructors which perform certain functionality on instantiation - e.g. register a factory method for a type in an another global collection of factory methods.