C++ Template Metaprogramming - Is it possible to output the generated code?

c++ templates metaprogramming seqan
n00ki3 picture n00ki3 · Aug 2, 2009 · Viewed 9.3k times · Source

I would like to debug some templated code to understand it better.
Unfortunately I'm new to template metaprogramming and it IS hard for me to get in.

When I try to output the preprocessed source files I get 125 000 lines of code :/

So is there a way I can see the generated Code? (The library I'm using is SeqAn)

Answer

jalf picture jalf · Aug 2, 2009

Nope, in general, it can't be done. Templates are simply part of the C++ language, they're not a separate preprocessor, so they don't generate C++ code.

The usual solution is to sprinkle your code with static asserts and other tests to verify that the right templates get instantiated in the right ways.

Once you start getting lost in your metaprogramming, this simple trick can help you determine which type a template parameter really is:

// given a variable t of an unknown type T
int*** i = t;

When the compiler encounters this, it'll print out a nice and simple error message, "Can not convert <long, detailed typename> to int***", allowing you to easily verify that the template parameter T is actually the type you think it should be.

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