variadic-templates

问题 Given a function f(x, y, z) we can bind x to 0, getting a function g(y, z) == f(0, y, z) . We can continue doing this and get h() = f(0, 1, 2) . In C++ syntax that would be #include <functional> #include <iostream> void foo(int a, long b, short c) { std::cout << a << b << c << std::endl; } int main() { std::function<void(int, long, short)> bar1 = foo; std::function<void(long, short)> bar2 = std::bind(bar1, 0, std::placeholders::_1, std::placeholders::_2); std::function<void(short)> bar3 = std

问题 What is the rationale of temp.res#8.3 (8) The validity of a template may be checked prior to any instantiation. [ Note: Knowing which names are type names allows the syntax of every template to be checked in this way. — end note ] The program is ill-formed, no diagnostic required, if: [..] (8.3) every valid specialization of a variadic template requires an empty template parameter pack, or That rule disallows trick as following to force template deduction as: template <typename ...Ts,

问题 I have a member function with a variable number of parameters, stored in a std::function , and I want to bind the instance and get an independent function object. template <class T, class R, class... Args> void connect(const T& t, std::function<R(const T&, Args...)> f) { std::function<R(Args...)> = /* bind the instance c into the function? */ } // ... Class c; connect(c, &Class::foo); For a fixed number of arguments I'd use std::bind , but I don't see how to do this for variadic parameters.

问题 I wonder what is the easiest and more common way to get the N-th parameter of a variadic templated class at compile-time (The returned value has to be as a static const for the compiler in order to do some optimizations). Here is the form of my templated class : template<unsigned int... T> MyClass { // Compile-time function to get the N-th value of the variadic template ? }; Thank you very much. EDIT : As MyClass will contain more than 200 functions, I can't specialize it. But I can

问题 I wonder what is the easiest and more common way to get the N-th parameter of a variadic templated class at compile-time (The returned value has to be as a static const for the compiler in order to do some optimizations). Here is the form of my templated class : template<unsigned int... T> MyClass { // Compile-time function to get the N-th value of the variadic template ? }; Thank you very much. EDIT : As MyClass will contain more than 200 functions, I can't specialize it. But I can

问题 I want a generic zipWith function in C++ of variable arity. I have two problems. The first is that I cannot determine the type of the function pointer passed to zipWith. It must be of the same arity as the number of vectors passed to zipWith and it must accept references to the vectors' element types respectively. The second is that I have no idea how to walk these vectors in parallel to build an argument list, call func(), and bail once the shortest vector is exhausted. template <typename R,

问题 I have the following function which can take N arguments of different types, and forwards them to N functions templated on each individual type, in this manner (example with two arguments): template <typename T1, typename T2> bool func(int& counter, T1 x1, T2 x2) { switch (counter) { case 0: if (func2<T1>(x1)) { counter++; return true; } else { return false; } case 1: if (func2<T2>(x2)) { counter++; return true; } else { return false; } default: return true; } } I want to write this function

问题 From [temp.variadic] (working draft) it seemed to me that a parameters pack can be expanded while defining an arguments list of another template class or function. Consider the following class: template<typename... T> struct S { template<T... I> void m() {} }; int main() { S<int, char> s; // ... } The intent is to capture the types used to specialize the template class S and use them to define an arguments list of non-type parameters for the member method m ( T is limited to a few types, of

问题 Say I have three classes, ClassA , ClassB , and ClassC . And all three of these classes have a function called updateValue(int) . And then I have controller class called, Controller . Who's constructor is templated like the following: class Controller { public: template <class...Classes> Controller(Classes & ...classes); // maybe initialize a tuple? void setValues(int val) { // unpack whatever data structure and call each classes updateValue(int) } private: std::tuple<Classes...classes>

问题 Here is a nice, succinct fold expression based lambda in C++17: #include <cstdint> using ::std::uint64_t; constexpr auto sumsquares = [](auto... n) { return ((n * n) + ...); }; // I want this to work. uint64_t foo(uint64_t x, uint64_t y, uint64_t z) { return sumsquares(x, y, z); } // And this too double bar(uint64_t x, double y) { return sumsquares(x, y); } I have this code I've written to do something similar in C++14, but it seems a lot more verbose and confusing than it should be. I'm