This question already has answers here:
Undefined reference to static variable [duplicate]
(2 answers)
Closed 9 years ago.
I am beginner to C++ and have a doubt about static member variables and member functions.
I have implemented a class as follows -
class Foo
{
private:
static int myVariable;
public:
static void setMyVariable()
{
myVariable = 100;
}
static void resetMyVariable()
{
myVariable = 0;
}
};
There are following considerations when I wrote a code like that -
I want only one instance of class Foo. Thats why I made all member variables and functions as static.
I don't want the outside code to touch myVariable
I have put this class in a header file and included in my main file. When I do this, I get an error undefined reference to Foo::myVariable
I want to know if I can write a code which can satisfy above requirements?
Thanks !
You need to define static class variables somewhere:
e.g. in your main C++ file,
int Foo::myVariable;
Note that technically, by making everything static, you may have no instances of Foo.
Related
This question already has answers here:
Undefined reference to static class member
(9 answers)
How to initialize private static members in C++?
(18 answers)
Closed 6 years ago.
I have a file A.hpp as such:
class A
{
private:
static std::string s;
public:
void modify_string();
};
I am implementing this in a file A.cpp as such:
#include "A.hpp"
void A::modify_string()
{
s = "something"; // Error here.
}
My main class:
int main()
{
A a;
a.modify_string();
}
I understand static variables are shared by all the class instances. I also went through this SO post where it says how to access the static member. Public static member of class . Could you please let me know where my concept is missing at?
Edit:
I am getting this error:
error: undefined reference to A::s
When you define:
void modify_string() {
s = "something"; // Error here.
}
You are creating a new function, not defining the member function modify_string of the class A. You need to do:
void A::modify_string() {
To inform the compiler that you are defining the member function modify_string for class A.
You also need a ; after your class definition.
Finally, the variable s is static so it needs to be defined seperatly somewhere so the linker can find a reference to it. So add:
std::string A::s = "default";
This was clearly described in the link you provided for your question.
Here is a working example: http://ideone.com/iQ6Kux
You need to reserve storage for s in exactly one compilation unit.
Do that by writing
std::string A::s;
In exactly one source file.
Your definition void modify_string() {...} in A.cpp is not defining the member function of the class, it's defining a separate global function with the same name. You probably meant
void A::modify_string()
{
s = "something";
}
This question already has answers here:
Undefined Reference to class static member in static member
(1 answer)
How to initialize private static members in C++?
(18 answers)
Closed 8 years ago.
Class base{
public :
static vector<int> _elems;
...
How can I use that static one.Must I define it out of the class body again?
Or I meet a trouble about a error"Undefine reference to 'base::_elems'"
You've only declared the static member, never defined it. In your cpp file you need to do this:
vector<int> base::_elems;
You can use it like any other variable. You only need to remember that the static variable is the same for all instances.
Edit: I forgot the defenition. You must define the variable, this can be done from any cpp file, but i recommend to define the variable in the file base.cpp.
This question already has answers here:
c++ calling a non static function from a static function
(3 answers)
Closed 8 years ago.
so I've got something like this:
//somewhere in the class header
static void bar();
Someobj baz;
//meanwhile in the implementation
void foo::bar()
{
baz.f()//this doesn't work b/c baz isn't declared as static
}
Do I have to declare everything I want to use in the static function foo as static also or is there a better way?
Yes you do. If you have 10 objects and they each have their own baz, then how is bar supposed to know which baz you're talking about?
It can't. That's why you can only have one baz as well, a static one.
Static functions are like global functions, there is no associated instance with them so you cannot refer to an instance member. If you need a single baz object associated with this class make it static also. Otherwise, you need to change your logic.
This question already has answers here:
Undefined reference to static class member
(9 answers)
C++ - Initialize and modify a static class member
(4 answers)
Closed 9 years ago.
I have problem with pthread_mutex_t. When I try to create static field pthread_mutex_t, then initialize it in static function and finally use it within some class methods I get many errors like:
main.o: In function `LogWriter::initialize(pthread_mutex_t*)':
main.cpp:(.text._ZN9LogWriter10initializeEP15pthread_mutex_t[LogWriter::initialize(pthread_mutex_t*)]+0x7): undefined reference to `LogWriter::mutex'
Simpplified class code:
class LogWriter{
static pthread_mutex_t mutex;
static void initialize(pthread_mutex_t *mut){
LogWriter::mutex = PTHREAD_MUTEX_INITIALIZER;
//if(pthread_mutex_init(&(LogWriter::mutex), NULL) != 0){
//init failed
//}
}
public:
static LogWriter getInstance(string module_name){
LogWriter instance(module_name);
return instance;
}
LogWriter& operator<<(string a);
};
My quesiton is: why ? I know that if I define it as normal (non-static) field I won't have any problems. Also searched google but I couldn't find any materials that are linked with this.
Also creating pointer to static pthread_mutex and initializing in in main function ends like this.
In some source file in your code, you need to add:
static LogWriter::pthread_mutex_t mutex;
The compiler won't "place" your variable in any particular source file, you have to do that for it. The class declaration just tells the compiler "I'll have a static variable somewhere" - but since, at least in theory, variables ordering and placement can make a difference [you may for example have different object files product "data" that goes into different sections of memory in some embedded system], the compiler won't be able to just throw it in any place it likes - that could be somewhere you don't want it.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Initializing private static members
Why I can't initialize non-const static member or static array in class?
It is strange to me. Why not assume there is a static field at the global scope?
It has to be placed somewhere (in some object file), so linker could find it. If you have declaration of class with static filed in .h file and include this file in a few .cpp files, then it would be ambiguous, which object file should have place allocated for this filed.
Please also note, that primitive type const static field could be initialized in class declaration:
class Foo
{
static const int n = 42;
};