Template undefined reference on windows with MinGW 4.8

Question

I am using static template members in a class; the templates are instantiated in a cpp built in a software. I have a plug-in for the software that uses the name() template method in the header, but does not build the source file which contains the instantiation. The build works on Linux with g++-4.9 but fails on MinGW 4.8. I want to know how to make it work with the almost same compiler, but on Windows instead.

.hpp :

enum class ToplevelMenuElement
{
    FileMenu, 
    ...
    AboutMenu
};

enum class FileMenuElement
{
    New,
    ... ,
    Quit
};

// Others menus macros are defined

class MenuInterface
{
    public:
        template<typename MenuType>
        static QString name(MenuType elt);

    private:
        static const std::map<ToplevelMenuElement, QString> m_map;
        static const std::map<FileMenuElement, QString> m_fileMap;
};

.cpp :

template<>
QString MenuInterface::name(ToplevelMenuElement elt)
{
    return m_map.at(elt);
}

template<>
QString MenuInterface::name(FileMenuElement elt)
{
    return m_fileMap.at(elt);
}

const std::map<ToplevelMenuElement, QString> MenuInterface::m_map
{
    {ToplevelMenuElement::FileMenu, QObject::tr("File")},
    ...
    {ToplevelMenuElement::AboutMenu, QObject::tr("About")}
};

const std::map<FileMenuElement, QString> MenuInterface::m_fileMap
{
    {FileMenuElement::New, QObject::tr("New")},
    ..., 
    {FileMenuElement::Quit, QObject::tr("Quit")}
};

Error :

undefined reference to `QString MenuInterface::name<ToplevelMenuElement>(ToplevelMenuElement)'

Is there any flag to use to make some kind of lazy instantiation ? Or should I build the .cpp containing the template instantiation in my plug-in ?

It's not useful to write a template method if its content is different according to the parameter(s) type. — vincentp, Nov 17 '14 at 18:37
@P0W This doesn't look like a duplicate since his code contains explicit specializations in the cpp file. — David G, Nov 17 '14 at 18:37
@vincentp it makes an interface easier to read (there would be ten different overloads else...) — Jean-Michaël Celerier, Nov 17 '14 at 18:38
@Jean-Michaël Celerier: I totally agree with you, but the interest of template programming is genericity^^ — vincentp, Nov 17 '14 at 18:40
@Well it does not cost me anything to do this (except the problem with this particular compiler), so why not ? — Jean-Michaël Celerier, Nov 17 '14 at 18:42
Also other people will be able to add their own types (at compile time) easily in their plugins with this way. — Jean-Michaël Celerier, Nov 17 '14 at 18:43

score 2 · Answer 1 · answered Nov 17 '14 at 18:42

2

Because you are linking a source file that contains explicit specializations, you need to declare your explicit specializations before you define them. From §14.7.3/3:

A declaration of a function template, class template, or variable template being explicitly specialized shall precede the declaration of the explicit specialization. [ Note: A declaration, but not a definition of the template is required. — end note ]

So you need to put these after your class in your header file:

template<>
QString MenuInterface::name(ToplevelMenuElement elt);

template<>
QString MenuInterface::name(FileMenuElement elt);

answered Nov 17 '14 at 18:42

David G

94,763
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@Jean-MichaëlCelerier Try adding an explicit instantiation in the source file (i.e `template QString MenuInterface::name(ToplevelMenuElement)`...) – David G Nov 17 '14 at 20:16
@Jean-MichaëlCelerier No, just `template QString MenuInterface::...`. Do it for both sepcializations. – David G Nov 17 '14 at 20:31
@Jean-MichaëlCelerier Did it work? If it didn't then I don't know any way to fix it other than to try putting the definitions in the header file. – David G Nov 23 '14 at 22:10

score 0 · Answer 2 · answered Nov 17 '14 at 18:32

0

This has been asked many times...declaration and definitions for templates should be same file.

answered Nov 17 '14 at 18:32

ravi

10,994
1
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36

It works just fine on Linux with gcc-4.9 so I hoped it would be the same on MinGW... – Jean-Michaël Celerier Nov 17 '14 at 18:33
Only general definitions. – chris Nov 17 '14 at 18:47

Template undefined reference on windows with MinGW 4.8

2 Answers2