template-meta-programming

问题 I would like to create template class that implements print() method for each type passed as template parameters. Something like that: class Interface { public: virtual ~Interface() = default; virtual void print(int) = 0; virtual void print(double) = 0; }; X x<int, double, Interface>; class X has public method void print() and it works. The whole code below: #include <iostream> #include <type_traits> struct Printer { void print(int i) {std::cout << i << std::endl; } void print(double d) {std:

问题 I'm using FLTK to do my GUI related stuff, and it requires functions of type void (*fn)( Fl_Widget*, void* ) to be registered as widget callbacks. I'm tired of creating function forwarders by hand that unpack the void* s into parameters and call appropriate static functions from my classes to do the work the user has requested. I came up with a solution, but it requires a class to be specialized for both function type, and function address, and this is where I'm having problems. Here's some

问题 Is it possible to store states in C++ metaprograms? I'm not entirely sure what I mean, but I want a template class that can be "assigned" to, e.g. state::value can be changed via the use of C++ templates. I know I probably sound very vague, but I hope someone understands what I'm talking about (also please no Boost.MPL). 回答1: It's not possible, the TMP part of C++ is a purely functional language without the concept of side effects. 回答2: That would be a heaven for bugs. It would also violate

问题 I was trying to reverse a c++14 std::index_sequence and ran into problems with my original implementation that used inheritance. I found a workaround using local type aliases, but I would like to understand why the original code does not work. Broken Reverse Using Inheritance This was my first attempt at reversing a std::index_sequence : /// Helper class that appends an element onto an index_sequence. /// Base case. template<size_t, typename> struct Append : std::index_sequence<> { };

问题 I am trying to write a template metafunction to detect if a type has a member function with void type. Currently I am able to use SFINAE to detect if a member function has a definite type such as double, int, etc. using something like template<typename C> static auto Test(void*) -> decltype(int{std::declval<C>().foo()}, Yes{}); And of course I can invert that (as shown in the attached code snippet) to test that it is not int, but I am unable to figure out how to test that it is void. The code

问题 I'd like to do exact this to get a list of types/classes. But I cannot use C++11. Any suggestion how I can append type to the template list? Edit: some code to what I'd like to do: #include <iostream> #include <typeinfo> #include <boost/mpl/vector.hpp> #include <boost/mpl/push_back.hpp> #include <boost/mpl/for_each.hpp> using namespace std; class A {}; class B {}; class C {}; typedef boost::mpl::vector<> type1; // supposed I'd like to have this as a #define macro so someone can call //

问题 I know that there are easier ways to do it, but I would like to initialize at compilation time the map from unrolled index of 2d array to its general format. I would like to do this without needing to instansiate the array object. Below I define the map from array[][]->array[] . Now I wonder how to do the opposite: [] -> [][] without hardcoding the chosen mapping scheme. I guess that should be possible using metaprogramming and variadic templates. But I tried using it for the first time just

问题 I'm using BOOST_FUSION_ADAPT_STRUCT() , and I need to check that all the members are declared and in the correct order. So first I did this: template <typename Sequence> struct checker { static void check() { typedef typename mpl::accumulate<Sequence, mpl::size_t<0>, mpl::plus<mpl::_1, mpl::sizeof_<mpl::_2>>>::type total_size; static_assert(sizeof(Sequence) == total_size::value, "omitted field?"); } }; And this works: struct foo { int x; float y; double z; }; BOOST_FUSION_ADAPT_STRUCT(foo, x,

问题 I use a three dimensional std::array , because the size is already known at compile. However, I noticed that the size() function is not static, and thus, inaccessible for constexpr/template functions. I already found the demo example below, which estimates the size of a one dimensional std::array . However, this does not work for two or more dimensions. Is there a way to return the other dimensions by writing a function with an additional template parameter dim for the x, y, z, .. dimension?

问题 I'm trying to deduce the greater of two template arguments at compile time. Both template arguments are of type size_t. I have a templated type, SomeType, which takes a size_t as it's template argument. I then have a function that takes two SomeType parameters with different template size_t's and i want the return type to be a SomeType with its templated size_t to be the greater of the two input size_t sizes. template <size_t d> struct SomeType {...} template<size_t d1, size_t d2> SomeType