July 2015 - Boost - lists.boost.org

Metaprogramming: parameter pack expansion/evaluation trick
by Antony Polukhin 03 Jul '15

03 Jul '15

Hi, Straight to the problem. Consider the situation, were a function accepts parameter pack and something need to be done with each argument (for example, we want to print each argument on a new line): template <typename... Args> void print(Args&&... a) { // print each a on a separate line } There's a known trick to do that: template <typename... Args> void print(Args&&... a) { (void) (int []){ 0, (std::cout << a << std::endl, 0)... }; } But the syntax is not very readable (and I always forget it and search for an example in Internet). I've tried to improve the syntax, and here's the result: template<typename... Args> void print(Args&&... a) { expand_if<Args...> { std::cout << a << '\n'... }; } Compete source could be found here: http://coliru.stacked-crooked.com/view?id=c2c59f22574671e5 Here's some more usage examples: template<typename... Args> void printer_int(Args&&... args) { expand { std::cout << std::abs(args) << std::endl..., std::cout << "Printing once again all the numbers: " << std::endl, std::cout << " Once again " << std::abs(args) << std::endl..., std::cout << "EOF Printing once again all the numbers." << std::endl }; } template<typename... Args> float sum(Args&&... args) { float res = 0; expand_if<Args...> { res += args... }; return res; } I hope this would be helpful for someone or it could be used as an argument for a proposal to extend C++ and allow simpler syntax: template<typename... Args> void print(Args&&... a) { std::cout << a << '\n'...; } P.S.: There was a nicer solution that newer required "_if<Args..>" : namespace detail { struct expand_impl final { template <class... Arg> constexpr inline expand_impl(Arg&&...) noexcept {} }; } template <class... Arg> using expand = detail::expand_impl; But unfortunately there's no way to force the compiler to evaluate expressions in constructor call from left to right. So it did not work on GCC (but work on Clang). -- Best regards, Antony Polukhin

4 4

[math][website] SVG graphics all missing?
by John Maddock 03 Jul '15

03 Jul '15

It looks like all the SVG graphics used by the Boost.Math docs are missing from the website - is this a release SNAFU? Thanks, John.

1 1

[winapi] Major changes in develop
by Andrey Semashev 02 Jul '15

02 Jul '15

Hi, I've just merged a largely reworked version of Boost.WinAPI to develop. It fixes a few bugs and also changes the implementation for better conformance with the C++ standard. Hopefully, it should fix compilation issues with clang on Windows. Another significant change is that Boost.WinAPI targets Windows Vista and later by default. As before, it is possible to explicitly select another version to target by defining BOOST_USE_WINAPI_VERSION. For example, to build Boost for Windows XP one would invoke: b2 define=BOOST_USE_WINAPI_VERSION=0x0501 If anyone has problems with this change, please let me know.

2 5

[optional] How to use optional to lazy construct obj?
by Mostafa 01 Jul '15

01 Jul '15

I'm interested in doing something along the lines of: lazy_adaptor<obj_type> lz(arg1, arg2, arg3, ...) where then lz.construct() then constructs an object of obj_type with ctor of arguments "arg1, arg2, arg3, ...". So far I have the following: (Am I on the right track?) #include <utility> #include <tuple> #include <string> #include <boost/optional/optional.hpp> template <typename... Args> struct ctor_forwarder; template <typename... Args> ctor_forwarder<Args...> make_ctor_forwarder(Args&&... args) { // Note: The "Args..." template parameter to ctor_forwarder determines how // the caller's arguments are temporarily stored. If it's an rvalue, it's // temporarily stored by value, else it's temporary stored by reference. return ctor_forwarder<Args...>(std::forward<Args>(args)...); } template <typename... Args> struct ctor_forwarder { //TODO: make ctor private. template <typename... A> explicit ctor_forwarder(A&&... args) : m_args( ::std::forward<A>(args)... ) {} /* template <typename Callable> auto forward_to(Callable && caller) -> decltype( caller(::std::forward<Args>(m_args)...) ) { return caller(::std::forward<Args>(m_args)...); } */ template <typename T> void forward_to(::boost::optional<T> & obj) { obj.emplace(::std::forward<Args>(m_args)...); } private: typedef ::std::tuple<Args...> args_t; private: args_t m_args; }; template <typename Obj, typename... Args> struct lazy_adaptor { Obj on_enter() { m_args_forwarder.forward_to(m_obj); } private: ::boost::optional<Obj> m_obj; ctor_forwarder<Args...> m_args_forwarder; }; template <typename Obj, typename... Args> lazy_adaptor<Obj, Args...> make_lazy_adaptor(Args&&... args) { return lazy_adaptor<Obj, Args...>(); } struct Sample1 { Sample1(int num, std::string str) {} }; int main() { auto df = make_ctor_forwarder(10, "Ten"); //ctor_forwarder<int, std::string> df(10, "Ten"); auto lazy_ctor = make_lazy_adaptor<Sample1>(10, "Ten"); return 0; }

1 0

Fwd: [SORT] Parallel Algorithms
by Francisco José Tapia 01 Jul '15

01 Jul '15

Hi Steven I had been revising and testing spreadsort, and the results are impressive. I have prepared the benchmarks and include spreadsort. Now the numbers are extracted from a file generated with a random number generator. In a few days I will put all in a git repository for a easy access As I had seen, you use boost::sort for to invoke to spreadsort. If you use this name, which name use for to invoke introsort? I think a good idea can be call the algorithms by their name. If we do this we have the next algorithm list : - spreadsort - spreadsort_integer - spreadsort_float - spreadsort_string - introsort - parallel_introsort - smart_merge_sort - parallel_stable_sort - sample_sort I think it's a simple and easy to understand, because with the name sort, many people think about an algorithm as introsort ( general , not specific for several data types) and not stable. What's your opinion ?

3 19