Re: [boost] [GSoC][MPL11] Post C++Now update

On 5/20/14, 1:34 AM, Matt Calabrese wrote:

On Mon, May 19, 2014 at 6:11 AM, Zach Laine wrote:

...

I was one of the people at the conference that tried to convince Louis to approach things this way. I wasn't the only one, and I can only speak for myself. The advantage to me is that I might have a library that I can use to do *both* compile-time only, and runtime+compiletime operations. Now that I have constexpr function and decltype(), I find the lines between compiletime and runtime operations to be sufficiently blurred that I don't draw the clear distinction I once did. For instance, in C++14, I can simply elide certain metafunctions I used to require to compute the return values of certain functions -- anything that can be done with a fold can be written in the form "auto foo (args) { return fold(some_fn, args); }". This makes me gravitate towards a solution that marries runtime and compile time as much as possible.

I, too, am in support of this and stand by it being an ideal, unless that ideal proves unfeasible to implement or use. Louis also noticed one huge advantage of this that cannot be understated -- if a new mpl is implemented using the automatic return type deduction approach, regardless of whether or not it is actually merged with fusion, it provides a very powerful way to debug metaprograms -- simply call the function without decltype but with the same parameters and you can then step through the code in your favorite debugger at runtime, including the ability to set breakpoints and step through line by line. Just make sure that you force your arguments to not be constexpr so that the evaluation happens at runtime for the sake of debugging.

Wow, that is awesome! I wanted to be able to do that for a long time! Regards, -- Joel de Guzman http://www.ciere.com http://boost-spirit.com http://www.cycfi.com/