Template subclass of non-template class - unresolved externals

Question

Possible Duplicate:
C++ template, linking error
Undefined symbol on a template operator overloading function

So I have an abstract Button class:

class Button
{
public:
    int x, y, width, height;
    std::string label, text;
    bool checkForClick(int mouseX, int mouseY);
    virtual void click(int mouseX, int mouseY) =0;
    virtual void draw(); //has a default implementation which can be overridden
};

and a subclass which I want to be a template class:

template <typename T>
class IncrementButton : public Button
{
public:
    T& controlValue;
    T increment;
    T minVal, maxVal;

    IncrementButton(T& controlValue) : controlValue(controlValue) {}

    void click(int mouseX, int mouseY);
    void draw(); 
};

The overridden methods look like:

template <typename T>
void IncrementButton<T>::click(int mouseX, int mouseY)
{
    //do something with controlValue
}

template <typename T>
void IncrementButton<T>::draw()
{
    //use value of controlValue
}

but this won't compile. It gives me the error:

Error   1   error LNK2001: unresolved external symbol "public: virtual void __thiscall IncrementButton<float>::click(int,int)" (?click@?$IncrementButton@M@@UAEXHH@Z)

...and the same thing for draw()

Any ideas? I'm relatively new to C++ so I'm hoping maybe there's something silly I'm doing wrong.

Are your overridden methods in the header file? Or the cpp file? IF they're in the cpp file, they need to be in the header file: http://stackoverflow.com/questions/1353973/c-template-linking-error — JaredC, Jan 05 '13 at 03:39
I suspect you need to read this: http://www.parashift.com/c++-faq/separate-template-fn-defn-from-decl.html. — Oliver Charlesworth, Jan 05 '13 at 03:41

score 0 · Accepted Answer · edited May 23 '17 at 11:48

0

You can fix your problem by moving your definitions into the header file. There are other solutions though too, but I'll let others explain them better-- see these links:

FAQ

Another Answer

edited May 23 '17 at 11:48

Community

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answered Jan 05 '13 at 03:53

JaredC

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The other solution, which is not actually explained by the answer linked here, is to use an explicit instantiation in the .cpp file where the template functions are defined. I think it's important to mention this, although putting all definitions into the header is typically the preferred solution. – jogojapan Jan 05 '13 at 03:59
@jogojapan Oli's link above mentions that, but I pointed to the more general FAQ topic. Thanks for pointing this out. – JaredC Jan 05 '13 at 04:10

Template subclass of non-template class - unresolved externals

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