How do I force a particular instance of a C++ template to instantiate?

c++ templates instantiation
anon picture anon · Jan 28, 2010 · Viewed 30.8k times · Source

See title. I have a template. I want to force a particular instance of a template to instantiate. How do I do this?

More specifically, can you force an abstract template class to instantiate?

I might elaborate as I have the same question. In my case I am building a library, some of the template implementations are large and include lots of stuff, but are only generated for a couple of types. I want to compile them in the library and export all the methods, but not include the header with the code everywhere.

ie:

template<class T>
OS_EXPORT_DECL class MyTmpl
{
    T *item1;
public:
    inline T *simpleGetT() { return(item1); } /* small inline code in here */ } 
    T *doSomeReallyBigMergeStuff(T *b); // note only declaration here
};

// *** implementation source file only seen inside library

template<class T>
MyTmpl<T>::doSomeReallyBigMergeStuff(T *b)
{
    ... a really big method, but don't want to duplicate it, 
        so it is a template ...
}

I could of course reference all the methods inside the library which would force them to compile and export but the desire isn't to add un-needed code to the library like the argument formatting for the items and the code to call them etc.

????? specifically I am building the library for several versions of MSC and GCC and intel compilers.

Answer

Georg Fritzsche picture Georg Fritzsche · Jan 28, 2010

You can't force generic templates to instantiate, the compiler can only generate code if the type is completely known.

Forcing an instantiation is done by providing all types explicitly:

template class std::vector<int>;

Comeaus template FAQ covers the related issues in some detail.

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