[C++-sig] function template instantiation

[Niall Douglas]

On 29 Mar 2012 at 16:14, Kyle Cronan wrote:

> So my question is, why isn't taking the address of some instance of a
> templatized function and passing it to def() enough to require the
> function definition to exist?  Should this be considered a usability
> bug in the library, a problem with my compiler, or maybe neither?

No, in that earlier post this behaviour is by design and intent (and 
I'm surprised no one said so at the time). When you do this:

    class_< field<vect> >("field_vect", init< int >())
        .def("getsize", &field<vect>::getsize)
    ;

... you're telling the compiler to instantiate field<vect>::getsize. 
Problem is, the compiler hasn't seen what getsize is supposed to be 
because its implementation was declared in a separate compilation 
unit. Therefore it can't know how to mangle the symbol for 
field<vect>::getsize, and therefore can't resolve the symbol.

Besides the above, you should NEVER declare template implementations 
outside a header file. Otherwise it easily leads to violations of the 
ODR. The old export keyword was supposed to solve the problem of 
having to declare template implementations in headers, but that's 
dead as a concept now.

BTW, regarding inlining, there are magic attributes to force never 
inlining. Very, very useful for debugging optimised code, and easier 
and clearer to use those than obfuscating the code.

HTH,
Niall

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