Is it possible in C++ to access to global template variable through non template function

Question

I have template class Application

It should be something like singleton, I want to create it once, and get from other files.

//main.cpp
Application<NetworkService, User, Policy> a;
a.run();
//other files
//instead of auto a = Application::getInstance<NetworkService, User, Policy>() I want just
auto a = Application::getInstance()

Is it possible? Maybe in another form, I just don't want to use template specification to access to created early global Application object

`using NetworkServiceApp = Application; ... NetworkServiceApp a; ...` Is that what you meant? Make it so you don't have to repeat the template parameters everywhere you mention the type? — doug65536, May 30 '16 at 13:46
Application is a part of a hpp library. Application withtemplate specification should be created in main.cpp and than users should access to the created instance using my library function without template arguments. And in my library I don't know what template arguments were used. — Sergey Kolesov, May 30 '16 at 14:03

Joey Yandle · Answer 1 · 2016-05-30T13:28:45.697

0

Add a class ApplicationBase, and have Application inherit from it. Put the singleton accessor in the base class, and add virtual functions for everything you want to call.

This way you will always interact with the base class, but you can use template args to construct it in main.cpp.

class ApplicationBase {
public:
    static ApplicationBase* getInstance() {
        return m_instance;
    }

    virtual void foo() = 0;

protected:
    static ApplicationBase* m_instance;
}

template<TNetworkService, TUser, TPolicy>
class Application : public ApplicationBase {
public:
    Application () {
        m_instance = this;
    }

    virtual void foo() {
        // do something
    }
}

Then outside main you can call

auto a = ApplicationBase::getInstance();
a->foo();

The constructor for Application will have to register the singleton in the parent class.

edited May 30 '16 at 13:28

answered May 30 '16 at 13:14

Joey Yandle

141
4

1

This is [PIMPL](http://stackoverflow.com/questions/60570/why-should-the-pimpl-idiom-be-used). This is overkill, a typedef would be sufficient to remove the requirement to pass template arguments. – doug65536 May 30 '16 at 13:28
Until C++11 there were no templated typedefs. – Joey Yandle May 30 '16 at 13:30
1

You don't need a templated typedef. A simple `typedef Application ApplicationImpl` is all you need (or `using ApplicationImpl = Application `). Then `ApplicationImpl` can be used in place of `Application ` – doug65536 May 30 '16 at 13:33
`typedef` could always *refer to* a template, the new thing was templating the typedef itself, using `using`. – doug65536 May 30 '16 at 13:42
I can use c++11 or c++14 it is not a problem. I think a simple typedef woldn't work. Application is a part of a hpp library. Application with specification should be created in main.cpp and than users should access to the created instance using my library function without template arguments. And in my library I dont know what template arguments was used. I guess idea with ApplicationBase should works, I'll try it. And what about templayted typedef, could I use it? – Sergey Kolesov May 30 '16 at 13:54
No, Ide with ApplicationBase doesn't work for me because there is method wich return type is template argument. – Sergey Kolesov May 30 '16 at 13:59

Is it possible in C++ to access to global template variable through non template function

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