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Subject: [boost] [forward_declare] Interest Inquiry toward Faster Compile Times
From: Daniel Larimer (dlarimer_at_[hidden])
Date: 2012-08-01 20:36:03

I have been increasingly frustrated with how long it takes to compile c++ code. After spending some time in Java and C# I can feel the pain of compile times much sharper than I use to. After seeing that Java and C# were able to achieve some level of generics without requiring 'everything in the header' and knowing that C compiles an order of magnitude faster than c++ I set about to find a way to increase my compile times.

One of the common recommendations is to 'forward declare' everything and to use the PIMPL (private implementation pattern). Unfortunately, PIMPL usually implies heap allocation and if you ever want to return-by-value you are stuck including the entire header for the types you use in your header.

I started inspecting some of my code and running it through the pre-processor and noticed that almost every source file was in the 50K+ lines of code post-preprocessor and that some files were north of 250K lines of code. This was ridiculously inefficient, especially because I was already using PIMPL everywhere I could and most of these lines were stl + boost.

After a little bit of work I was able to abstract vector, boost::exception, iostream, string, threading, and many of the other types that I had been including that resulted in 'code-bloat'. The resultant library was able to reduce my post-preprocessor output to just one or two thousand lines of code (assuming the .cpp itself was 500+ lines). My compile times dropped to sub-second per object file from 10+ seconds (some up to 40 seconds).

There are some side effects from doing this, performance in some cases was slightly slower due to one level of indirection, but with with link-time optimization this performance loss was minimized. And any loss of performance was clearly acceptable trade for the gain in development efficiency. In situations were performance really is critical I can choose to include the raw header in individual cpp files, but in most cases that level of 'optimization' is overkill.

One of the things I realized doing this work was that often the only reason you need to include a header is because the compiler needs to know the SIZE of the type that you wish to forward declare and nothing else. So our development is slowed entirely due to the lack of a SINGLE NUMBER. So I created a utility that allowed me to 'forward declare' the size of the variable. This allowed me to use return by value and member variables via forward declaration WITHOUT having to resort to heap allocation.

It occurred to me that a library that allowed developers to choose a little drop in runtime performance for a dramatic improvement in compile times would be generally useful to the C++ community.

I have included an example of the 'technique' and general purpose utility that would allow others to quickly and easily integrate forward declaration of 'value types'.

A full up boost submission would probably include pre-wrapped versions of most of the STL and Boost types.

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