Debugging C++ template metaprograms

Structure

• Template metaprogramming (TMP)

• Debugging– In general– TMP

• Templight framework– Idea– implementation

• Results

Template metaprogramming

• C++ generics are Turing complete, we can implement algorithms that run in compile time

• Factorial example

Comparison

• Run time vs. compile time

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TMP ontology 1

• Ill-formed and well-formed programswell-formed = according to language rules

TMP ontology 2

Debugging in general

Debugging TMP

Today’s possibilities

• Compiler support (preferred long term solution)

• Language elementsSince we don’t have compiler support, use the existing language elements and the analogue between run time and metaprograms

If we had a trace file?

• We can debug a program a posteriori if we have a trace of the run

• Therefore if we had such a trace file for template metaprograms then we would be able to debug them

Warnings are talkative

• Today’s compilers usually display warning messages that contain essential contextual information

• Artificially generate such warnings

Portable warning

• Consider the following line:…

Build outputsG++ Watcom

MSVC 7.1, 8.0 Comeau

Etc.

Step #1: preprocessing

• Use the existing preprocessor to obtain a single source file

Step #2: instrumentation

• Insert our specially designed warnings to the interested positions

Step #3: Build -> warnings

• Feed the compiler with this modified source and save the warnings

Step #4: warnings -> trace file

• Gather the information from the warnings and create a trace file for further use

Results: TMP debugger IDE

• How it looks like in real

Ideal compiler support

• Special output to migrate these hacks into the compiler

Limitations

• Different levels (no output, only partial info)

• Simple pattern matching is not strong enough, ambiguity

Related work

• Profiling

• TMP book efficieny measurements

• Tracer

• Mpl::print

• archetype

Future directions

• Eliminate limitations

• Profiling

• IDE integration

• Compiler support

Conclusion

• This is a prototype that works with limitations

• With compiler support useful tools could be easily created

Contact

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