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.
Related
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:
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.
This question already has answers here:
How to initialize private static members in C++?
(18 answers)
Closed 9 years ago.
So I have the following c++ class
class MyClass:
public:
static void InitObject();
private:
static MyObject *myObject;
};
And then in the .cpp file I do
void MyClass::InitObject
{
myObject = new MyObject();
}
However, I get a compiler error saying that "myObject" was referenced from InitObject() and then it says Linker command failed with exit code 1.
Why doesn't this work? How do I fix it?
Since static data members of classes in C++ need memory space that is separate from the instance memory, they need to be defined, in addition to being declared in the class.
You do that in a separate translation unit (in your CPP file) like this:
MyObject *MyClass::myObject;
This definition tells the compiler to allocate space for myObject in the static memory area. Without this definition, the code is going to compile, but the linker will report an error, because it is responsible for ensuring that all referenced static objects have memory allocated to them.
Extend your .cpp file with the following definition for myObject:
MyObject* MyObject::myObject = NULL;
NOTE:
For your particular case you might be better off saying:
class MyClass:
{
public:
static MyClass& instance();
private:
MyClass() {}
};
and in .cpp file:
MyClass& MyClass::instance()
{
static MyClass myInstance;
return myInstance;
}
I'd prefer this over using new MyClass(). Any access to the instance() method will guarantee your instance is initialized exactly once, and you'll get a valid reference to it.
'Space' is completely allocated on the stack then, as well for the reference as for the instance itself.
You never allocated space for MyObject::myObject. Put this in the CPP file:
MyObject* MyObject::myObject;
This question already has answers here:
What does it mean to have an undefined reference to a static member?
(2 answers)
Closed 9 years ago.
I have a .cpp file that looks something like this:
//other code
namespace {
class C1;
class C2;
class C2{
public: static int counter;
//member functions here
};
class C1{
//other code
C2::counter = 10;
};
}
When I run 'make' I get the following error:
relocation R_386_GOTOFF against undefined symbol '(anonymous namespace)::C2::counter' can not be used when making a shared object...
Am I missing something simple here? Shouldn't the static int be available for class C1 to change it? Also, I am developing this as a part of the Clang library's. Also, I can share the Makefile if that helps.
You have missed out on providing the definition of you static variable. This definition must occur outside the class and only one definition is allowed. Usual way to do this is to provide the definition in the implementation file.
Because you are directly using it, without providing any definition for it, you are getting the error.
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;
};