variadic

问题 Can you see anything wrong with this function declaration? template<typename... Containers> std::tuple<typename Containers::value_type...> foo(const Containers &...args); When I try to call it, like this: foo(std::list<int>(), std::vector<float>()); MSVC2013 says error C2027: use of undefined type 'std::tuple<Containers::value_type> . I tried rewriting the function declaration with the "late return" syntax and it made no difference. Is there any way I can achieve what this code is trying to

问题 Is there any way at all for this code to compile and work as intended without resorting to va_list stuff ? #include <iostream> void fct(void) { std::cout << std::endl; } void fct(int index, int indexes...) { std::cout << index << ' '; fct(indexes); //or fct(indexes...); ? } int main(void) { fct(1, 2, 3, 4, 5, 6, 7); return 0; } 回答1: I suspect you have misunderstood the meaning of the signature void fct (int index, int indexes...) I suspect you think that fct() expect a int single value (

问题 I have some functions that read various types from serialized data, eg: class DataDeserializer { int getInt(); std::string getString(); MyClass getMyClass(); } I then have various callback functions that take arbitrary parameters, eg: void callbackA (int, int, int); void callbackB (int, std::string); void callbackC (std::string, int, MyClass, int); I want to call the various callbacks with arguments read from the deserialized data stream. What I would like is to automate the boilerplate code

问题 Suppose I have a variadic template function like template<typename... Args> unsigned length(Args... args); How do I find the length of the parameter list using the length function ? 回答1: Use sizeof... : template<typename... Args> constexpr std::size_t length(Args...) { return sizeof...(Args); } Note you shouldn't be using unsigned , but std::size_t (defined in <cstddef> ). Also, the function should be a constant expression. Without using sizeof... : namespace detail { template<typename T>

问题 I have a class template and I can't seem to figure out how to perform a Variadic Template style instantiation. Here is the "code" so far of what I'm looking for: template<typename _Classname, typename... Args> class CFunctorStartExT { friend class CXXFactory; protected: template<typename U> CFunctorStartExT(typename U& _functor, Args&... args) : m_Functor(_functor), m_args(args) { } virtual bool ProcessLoop(CSomeClass* pThread) { return m_Functor(pThread, m_args); } protected: _Classname& m

问题 I've prepared a simple variadic template test in Code::Blocks, but I'm getting an error: No matching function for call to 'OutputSizes()' Here's my source code: #include <iostream> #include <typeinfo> using namespace std; template <typename FirstDatatype, typename... DatatypeList> void OutputSizes() { std::cout << typeid(FirstDatatype).name() << ": " << sizeof(FirstDatatype) << std::endl; OutputSizes<DatatypeList...>(); } int main() { OutputSizes<char, int, long int>(); return 0; } I'm using

问题 How can variadic functions like printf find out the number of arguments they got? The amount of arguments obviously isn't passed as a (hidden) parameter (see a call to printf in asm example here). What's the trick? 回答1: The trick is that you tell them somehow else. For printf you have to supply a format string which even contains type information (which might be incorrect though). The way to supply this information is mainly user-contract and often error-prone. As for calling conventions:

问题 Let's assume that I have these macros that are prefixed with ATTRIB_ #define ATTRIB_A "a" #define ATTRIB_B "b" #define ATTRIB_C "c" I would like to be able to use a variadic macro that unpacks each given parameter then prefixes it with ATTRIB_ to obtain the full name of an attribute macro in order to expand that macro: #define ATTRIBS(...) CONFUSED_HERE(##__VA_ARGS__) Which would be used as: const char * a = ATTRIBS(A, B, C); And the result would be equal to: const char * a = "a" "b" "c"; As

问题 I want to create a function apply that takes a function with an arbitrary amount of arguments as well as a list of integers, and returns the result of the function (Where each integer in the list is an argument in order. I was thinking something like: apply :: ([Int] -> Int) -> [Int] -> Int apply f x:xs = apply (f x) xs apply f [] = f But I know this won't work because the type signature is wrong - the function doesn't take a list of ints, it just takes some amount of int arguments.

问题 I have the following question, I really can't compile from all the questions and articles researched: In C++, is it possible to have a method with variadic template arguments that specify types of arguments (as a meta-description type for parameters of in, out, in/out of a certain type, to be passed by value, by address etc.), to loop through these variadic arguments in order to instantiate variables of specified types, and be passed these variables to functions specified by a pointer in a