Cannot access public static variable member [duplicate] - c++

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linker cannot find a C++ static member [duplicate]
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I've declared a public static member to keep the total count of the instances of my class. The code is as follows:
class Hello {
public:
static int myCount;
void test(){
//do nothing
};
Hello(){
Hello::myCount += 1;
};
~Hello() {
Hello::myCount -= 1;
}
};
int main(int argc, const char * argv[]) {
// insert code here...
Hello *p1 = new Hello();p1->test();
Hello *p2 = new Hello();p2->test();
cout << Hello::myCount;
return 0;
}
However, I when compile, it says:
Undefined symbols for architecture x86_64:
"Hello::myCount", referenced from:
_main in main.o
Hello::Hello() in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I don't know where I'm wrong. It's been years from the last time I worked with c++, so could you please suggest a solution?
Thank you.

Answer 1

Static members have to be defined outside of the class, e.g.:
class Hello {
public:
static int myCount;
void test(){
//do nothing
};
Hello(){
Hello::myCount += 1;
};
~Hello() {
Hello::myCount -= 1;
}
};
int Hello::myCount = 0; // definition outside of the class
(...)
Here is an example to show, that it helps to solve your problem: http://ideone.com/LVXVCc
It's all because a rule called One Definition Rule.
You can read more about this one in a context of static class members here.
In short: static int myCount declaration is not a definition of a member. Classes are usually placed in their .h/.hpp header files and are included to many other files. If those'd contain static member and lines like the one above would be a definition, it will lead to the multiple-definitions error.
To prevent that, this declaration is not treated as a definition and you must define it yourself later.