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;
Related
This question already has answers here:
C++ singleton vs. global static object
(8 answers)
Closed 5 years ago.
I know that singleton allow only one instance of an object. Each method declared in the singleton will only operate on this object.
I was wondering why to not simply declare a global object which will achieve the same goal?
I am certainly forgetting something. If singletons exist there must be specific uses or help to realize specific mechanisms.
For instance:
class Singleton
{
public:
static Singleton& Instance()
{
static Singleton sg;
return sg;
}
void function();
};
would be the same as:
class NotSingleton
{
public:
NotSingleon();
~NotSingleton()
void function();
};
NotSingleton nsg;
However, nothing prevent me to use more than one instance of NotSingleton
Singleton is used when we do not want to create more than one object. Singleton class ensures that not more than one object is created. However, having a global object doesn't ensure this.
class Singleton {
public static Singleton object = null;
public void singleton() {
if (object == null)
object = new Singleton();
return object;
}
}
This class will not create more than one object. This is the purpose of the Singleton class.
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 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:
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:
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.