Static Variable Undefined Reference

Question

I'm trying to make sure that a module only loads exactly once but when I do it my way, the compiler spits out "Undefined reference to both my static class variables :S

class Text : public Parent { private: static int Instances; static HMODULE Module; public: Text(); Text(Text&& T); Text(std::wstring T); ~Text(); virtual Text& operator = (Text&& T); }; Text::Text() : Parent() {} Text::~Text() { if (--Instances == 0) FreeLibrary(Module); // Only free the module when // no longer in use by any instances. } Text::Text(Text&& T) : Parent(std::move(T)), Module(std::move(T.Module)) Text::Text(std::wstring T) : Parent(T) // Module only loads when // this constructor is called. { if (++Instances == 1) { Module = LoadLibrary(_T("Msftedit.dll")); } } Text& Text::operator = (Text&& T) { Parent::operator = (std::move(T)); std::swap(T.Module, this->Module); return *this; }

Any ideas why it says undefined reference to BOTH variables (Instances & Module)?

Are the definition and implementation of the class in the same file or in separate files? Where does the error message point to? — gongzhitaao
– gongzhitaao, Commented Mar 24, 2013 at 2:57
It points to the Cpp file that contains the definition for the class. The header and cpp file are separate. Gives no line numbers. — Brandon
– Brandon, Commented Mar 24, 2013 at 3:01
possible duplicate of Undefined reference to static variable — jogojapan
– jogojapan, Commented May 30, 2013 at 14:37

gongzhitaao · Accepted Answer · 2013-03-24 15:17:35Z

1

You should define your static variable in your before you can use it.

Add

int Text::Instancese = 0 // whatever value you need.

at top of your .cpp file.

edited Mar 24, 2013 at 15:17

answered Mar 24, 2013 at 3:01

gongzhitaao

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4 Comments

Brandon Over a year ago

Wow.. That worked.. Why though :S I don't understand why you'd declare a variable in the implementation file. I've never seen this before.

gongzhitaao Over a year ago

@CantChooseUsernames This is C++:) I think any introduction on C++ has an explanation :D google it.

Konrad Rudolph Over a year ago

@CantChooseUsernames You don’t declare it – you define it.

gongzhitaao Over a year ago

@KonradRudolph Thanks for the wording.

Pete Becker · Accepted Answer · 2013-03-24 12:54:30Z

The class definition declares two static data members, Text::Instances and Text::Module. You also have to define those data members if the code actually uses them. Just like void f(); declares a function, but you can't call it unless you also define it. So add:

int Text::Instances; HMODULE Text::Module;

to your source code. (This has nothing to do with whether your current code is written entirely in a header or split into header and source; if you use it you have to define it)

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