C++ :: Static Member Allocation - Global Object Creation
Jan 24, 2014
I see many time where static data member is used to count creations of objects -
i.e.
1. the static data member is init to 0
2. the static data member is incremented by 1, in the Class' constructor, every time an object is created
However, if you define a global object of a class,
How can you tell that the static data member is initialized BEFORE the constructor of the global object is called? (i.e. before the global object is created).
Because to my understanding, you do not know in advance the order of global objects' creation -
so the Global Object could be created BEFORE the static data member was created and initialized.
Code: class A { std::map<std::string, Unit*> aMap; class B
[Code] .....
This code snippet results in "A non-static member reference must be made relative to a specific object". When I make callA() static, this error goes away, but there is problem with aMap.
What are the workarounds for accessing the non-static member variables of some class(Say A) inside static member functions of another class(Say B)? I am coding in c++. Class A is derived with public properties of class B. Any pointers?
Is it possible to use & change global variables in a Static Library? For example:
I declare a bool test = true; globally.
Then later in an exported function If the user wants, he can set that test to false. So the program later when checks test if it's true, will notice that it's not true, since one of my function changed it.
I've simplified things slightly - but the basic point is that both functions are in the same source file and they both have a static std::string called 'x'. Being static, I guess they aren't (strictly) local variables. So how does the compiler know that they're different entities? Does it encode their signatures using the function name or something like that? If I call each function separately I do seem to get the correct string...
enum Country {India, China, France, NumCountries}; // plus many other countries struct School {}; struct Mall {}; struct HockeyArena {};
[Code] ....
Output:
PersonFactory::initializeEthnicNames() called Carrying out the initialization... PersonFactory::initializeEthnicNames() called PersonFactory::initializeEthnicNames() called PersonFactory::initializeEthnicNames() called PersonFactory::initializeEthnicNames() called numberOfTimesInitialized = 1
As you can see, even though five PersonFactory objects were constructed, the ethnicNames initialization only occurred once, as desired. However, there are some issues with my method. First of all, the use of the comma operator is ugly in my opinion. But fashion statements aside, PersonFactory::initializeEthnicNames() is still called multiple times, which is not good, even though it correctly avoids reinitializing ethnicNames after the first call. Also, I now forever get the annoying compiler warnings that the bool namesInitialized is never used, which is true, thus wasting a small bit of memory. And finally, I cannot declare ethnicNames const now, and it is supposed to be const. Any better way to accomplish what I'm trying to do?
By the way, the reason why I don't initialize ethnic names outside the class as is normally done for static data members (and that would indeed allow me to declare it const) is because it would get messed up if I later change the order of the elements in enum Country. Hence actual lines of initializations I think are needed. And I do want ethnicSurnames inside PersonFactory, because I feel it really does belong there. Also, PersonFactory is not to be Singleton, because it has data members that depend on some parameters in its constructor (e.g. geographic location).
I am having a problem concerning a static const member variable I want to use to set a certain property of my class during development time. The question actually concerns proper implementation as I do have a solution that "works" at least. The variable should denote the size of a member array which I don't want to allocate on the heap due to serious performance issues. So here is my code:
//MyClass.h class MyClass{ public: static const int MyArraySize = 256; private: int MyArray[MyArraySize]; };
This works but it's not nice for two reasons:
1) It doesn't separate interface from implementation. I would prefer to define the variable in the corresponding .cpp file but it doesn't work:
//MyClass.h class MyClass{ public: static const int MyArraySize;
[Code] .....
If I delete the line int MyArray[MyArraySize]; the above code works but when I use it to define the size of the array I get a "constant expression expected" error for the line int MyArray[MyArraySize]; which makes sense as the compiler does not know the value of MyArraySize when he reaches int MyArray[MyArraySize]; and therefore can not allocate the memory. Of course I can move MyArray to the heap like that:
//MyClass.h class MyClass{ public: static const int MyArraySize; static const int MyValue;
[Code] .....
But as I mentioned before this causes a remarkable loss of performance.
Something like the following does not work:
//MyClass.h class MyClass{ public: static const int MyArraySize = (int) pow(2, 8); private: int MyArray[MyArraySize]; };
This gives a "constant expression expected" error for the line static const int MyArraySize = (int) pow(2, 8);
Interestingly the following code works:
//MyClass.h class MyClass{ public: static const int MyValue; };
//MyClass.cpp #include "MyClass.h" const int MyClass::MyValue = (int) pow(2, 8);
So if I use pow outside of the class definition I get no errors. Is there any solution to those problems? So what I want is: 1) Don't allocate the array on the heap 2) Separate interface from implementation 3) Being able to use functions like pow to define MyArraySize 4) Not use global variables
I am getting this error invalid use of non static data member.my code looks something like this: i have a main.cpp and 2 class files with respective .h files, say one class file is named human (so i have human.cpp and human.h) and stats (so i have stats.cpp and stats.h) in my stats.h file, i have a double array: double HumanF[10][12] with everything filled in.then in my human.h file i just have a bunch of integers. human.cpp has formulas in it that use numbers from the double array i mentioned. for example
Human::Human() { constant (this is a double i made in human.h) = (1+Stats::HumanF[0][0]); i (another double) = pow(constant, ylvl); (ylvl is also an int I made in my header file) yhp = i*137; }
How does one use a functor as a static constexpr member? I had this basic functor for a class:
struct functor{ short operator()(char c)const{return c-'0';} };
And in the class, I use it as a static constexpr member: class Foo{ public: //... private: static constexpr functor k_funky = functor(); };
During the linking stage, I kept getting "undefined reference to 'Foo::k_funky'". So then I tried declaring the functor's constructor and operator function constexpr:
I need to keep a static variable in a member function of a class that I have many objects of. I've had some trouble with it, and when I read up I found that such variables are static across all instances. Is there any way around this?
struct Speaker { static int numElem; string name; int number; // Phone string topic; float fee; };
// IN main() FUNCTION Speaker s[10];
The goal is for numElem to keep track of how many of the 10 elements are in use. However, I'm not sure the proper way to access the element, if it's even possible.
What am I doing wrong with static members and methods here?
compiler errors:
1>test.obj : error LNK2005: "private: static int Test::count" (?count@Test@@0HA) already defined in main.obj 1>c:usersjamesdocumentsvisual studio 2013Projectsstatic_testReleasestatic_test.exe : fatal error LNK1169: one or more multiply defined symbols found test.h #ifndef TEST_H_ #define TEST_H_ class Test {
What is the problem with the following code is? It compiles with Visual C++ 2012 but does not with g++:
//a.h
#ifndef Loaded #define Loaded using namespace std; class MyClass{ public: static const int MyStaticValue = 200;
[Code] ....
If I try to compile this using the command
g++ a.cpp b.cpp
I get an "undefined reference to 'MyClass::MyStaticValue'" error for the line "A = MyClass::MyStaticValue;" in main(). The strange thing is that if I change the line to "A = (int) MyClass::MyStaticValue;" it works fine and the output is
200 200
as expected.
The code also compiles under g++ if I move the defintion of MyStaticValue from a.h to a.cpp by const int MyClass::MyStaticValue = 200;
Would each instance of Foo create a new counter variable, or would it remain the same for all of them, i.e. baz.funky() would always use the same counter variable? What if the class was a template?
"You cannot initialize the static data member in the class definition — that’s simply a blueprint for an object and initializing values for members are not allowed. You don’t want to initialize it in a constructor, because you want to increment it every time the constructor is called so the count of the number of objects created is accumulated."
Why don't you want to initialize it in a constructor?
Edit: Because every time it is called it will set it back to 0 or whatever the initializing value.
For example, in a header file A.h, I define an abstract class,
Code:
// A.h class A { public: virtual void foo() = 0; private: static int _x; };
How'd I initialize static member data _x?Normally, we initialize a static member data in a cpp file. However, there is not cpp file for A.h. If I intialize _x in header file, there will be linker errors like mulitple defined symbols. What is appropriate way to do that?
I have a class having static member.I have get and set methods which will Get and Set Values to this variable. In a multithreaded application does it have any thread safety issues.
Class a { static int b; void Set (int c); int Get(); };
