Variadic Template，没有匹配的函数调用 [英] Variadic Template, no matching function call

问题描述

我正在写一个 zip 的实现，但我遇到了一个问题。这是一个最小的测试用例：

  #include< iostream& 
 #include< deque> 
 #include< tuple> 
 #include< string> 
 #include< limits> 
 
 template< template< typename ...> class Container，typename ... Types> 
 Container< std :: tuple< Types ...>> zip（Container< Types> const& ... args）{
 unsigned len = commonLength（args ...）; 
 Container< std :: tuple< Types ...>> res; 
 std :: tuple< Types ...>项目; 
 
 for（unsigned i = 0; i< len; i ++）{
 item = getTupleFrom（i，args ...） 
 res.push_back（item）; 
} 
 
 return res; 
} 
 
 template< class ContainerA，class ... Containers> 
 unsigned commonLength（ContainerA first，Containers ... rest，unsigned len = std :: numeric_limits< unsigned> :: max（））{
 unsigned firstLen = first.size（）; 
 if（len> firstLen）{
 len = firstLen; 
} 
 return commonLength（rest ...，len）; 
} 
 
 template< class ContainerA> 
 unsigned commonLength（ContainerA first，unsigned len = std :: numeric_limits< unsigned> :: max（））{
 unsigned firstLen = first.size（）; 
 if（len> firstLen）{
 len = firstLen; 
} 
 return len; 
} 
 
 template< template< typename ...> class Container，typename TypeA，typename ... Types> 
 std :: tuple< TypeA，Types ...> getTupleFrom（unsigned index，Container< TypeA> const& first，Container< Types> const& ... rest）{
 return std :: tuple_cat（std :: tuple< TypeA>（first [index]），getTupleFrom （index，rest ...））; 
} 
 
 template< template< typename ...> class Container，typename TypeA> 
 std :: tuple< TypeA> getTupleFrom（unsigned index，Container< TypeA> const& first）{
 return std :: tuple< TypeA>（first [index]）; 
} 
 
 int main（）{
 
 std :: deque< int> test1 = {1，2，3，4}; 
 std :: deque< std :: string> test2 = {hihi，jump，queue}; 
 std :: deque< float> test3 = {0.2，8.3，7，123，2.3}; 
 for（auto i：zip（test1，test2，test3））{
 std :: cout< std :: get< 0>（i）<< std :: get< 1>（i）<< std :: get< 2>（i）<< std :: endl; 
} 
 //预期输出：
 //1hihi0.2 
 //2jump8.3 
 // 3queue7 
 return 0; 
} 
  

编译时出现以下错误：

 错误：没有匹配的函数调用'commonLength（const Star :: List< int>& const Star :: List< std :: basic_string< char> ;&; const Star :: List< float>&）'
注意：候选是：
 note：template< class ContainerA，class ... Containers> unsigned int Star :: commonLength（ContainerA，Containers ...，unsigned int）
注意：template< class ContainerA> unsigned int Star :: commonLength（ContainerA，unsigned int）
  

我假设我指定了模板参数错误或类似的东西。我也试图重组和完全消除该功能，但后来我得到与 getTupleFrom 相同的错误。

任何人向我解释为什么我是愚蠢的？因为我只是不知道我做错了什么。 ：

 
  #include< iostream> 
 #include< deque> 
 #include< tuple> 
 #include< string> 
 #include< type_traits> 
 #include< algorithm> 
 #include< limits> 
 
 template< class ContainerA> 
 unsigned commonLength（unsigned len，const ContainerA&第一个）{
 unsigned firstLen = first.size（）; 
 if（len> firstLen）{
 len = firstLen; 
} 
 return len; 
} 
 
 
模板< class ContainerA，class ... Containers> 
 unsigned commonLength（unsigned len，const ContainerA& first，const Containers& ）{
 unsigned firstLen = first.size（）; 
 if（len> firstLen）{
 len = firstLen; 
} 
 return commonLength ...）; 
} 
 
 template< template< typename ...> class Container，typename TypeA> 
 std :: tuple< TypeA> getTupleFrom index，Container< TypeA>常数& first）{
 return std :: tuple< TypeA>（first [index]）; 
} 
 
 template< template< typename ...> class Container，typename TypeA，typename ... Types> 
 std :: tuple< TypeA，Types ...> getTupleFrom（unsigned index，Container< TypeA> const& first，Container< Types> const& ... rest）{
 return std :: tuple_cat（std :: tuple< TypeA>（first [index]），getTupleFrom （index，rest ...））; 
} 
 
 template< template< typename ...> class Container，typename ... Types> 
 Container< std :: tuple< Types ...>> zip（Container< Types> const& ... args）{
 unsigned len = commonLength（std :: numeric_limits< unsigned> :: max（），args ...）; 
 Container< std :: tuple< Types ...>> res; 
 std :: tuple< Types ...>项目; 
 
 for（unsigned i = 0; i< len; i ++）{
 item = getTupleFrom（i，args ...） 
 res.push_back（item）; 
} 
 
 return res; 
} 
 
 int main（）{
 
 std :: deque< int> test1 = {1，2，3，4}; 
 std :: deque< std :: string> test2 = {hihi，jump，queue}; 
 std :: deque< float> test3 = {0.2，8.3，7，123，2.3}; 
 for（auto i：zip（test1，test2，test3））{
 std :: cout< std :: get< 0>（i）<< std :: get< 1>（i）<< std :: get< 2>（i）<< std :: endl; 
} 
 //预期输出：
 //1hihi0.2 
 //2jump8.3 
 // 3queue7 
} 
  

它输出完全符合您的期望。问题是：







• 您未在<$中使用 const& 容器c $ c> commonLength ，而 zip 是const引用的参数。


• commonLength coudln't被推导，所以我把它移动到开头


• 你声明/定义函数的顺序错误

显然，clang 3.1无法推导出<$中的模板参数c $ c> zip ，但g ++ 4.6让它们很好。

I'm writing an implementation of zip, but I've ran into a bit of a problem. Here's a minimal test case:

#include <iostream>
#include <deque>
#include <tuple>
#include <string>
#include <limits>

template <template <typename...> class Container, typename... Types>
Container<std::tuple<Types...>> zip(Container<Types> const&... args) {
  unsigned len = commonLength(args...);
  Container<std::tuple<Types...>> res;
  std::tuple<Types...> item;

  for (unsigned i=0; i<len; i++) {
    item = getTupleFrom(i, args...);
    res.push_back(item);
  }

  return res;
}

template <class ContainerA, class... Containers>
unsigned commonLength(ContainerA first, Containers... rest, unsigned len=std::numeric_limits<unsigned>::max()) {
  unsigned firstLen = first.size();
  if (len > firstLen) {
    len = firstLen;
  }
  return commonLength(rest..., len);
}

template <class ContainerA>
unsigned commonLength(ContainerA first, unsigned len=std::numeric_limits<unsigned>::max()) {
  unsigned firstLen = first.size();
  if (len > firstLen) {
    len = firstLen;
  }
  return len;
}

template <template <typename...> class Container, typename TypeA, typename... Types>
std::tuple<TypeA, Types...> getTupleFrom(unsigned index, Container<TypeA> const& first, Container<Types> const&... rest) {
  return std::tuple_cat(std::tuple<TypeA>(first[index]), getTupleFrom(index, rest...));
}

template <template <typename...> class Container, typename TypeA>
std::tuple<TypeA> getTupleFrom(unsigned index, Container<TypeA> const& first) {
  return std::tuple<TypeA>(first[index]);
}

int main() {

  std::deque<int> test1 = {1, 2, 3, 4};
  std::deque<std::string> test2 = {"hihi", "jump", "queue"};
  std::deque<float> test3 = {0.2, 8.3, 7, 123, 2.3};
  for (auto i : zip(test1, test2, test3)) {
    std::cout << std::get<0>(i) << std::get<1>(i) << std::get<2>(i) << std::endl;
  }
  //expected output:
  //1hihi0.2
  //2jump8.3
  //3queue7
  return 0;
}

When compiling I get the following error:

error: no matching function for call to ‘commonLength(const Star::List<int>&, const Star::List<std::basic_string<char> >&, const Star::List<float>&)’
note: candidates are:
note: template<class ContainerA, class ... Containers> unsigned int Star::commonLength(ContainerA, Containers ..., unsigned int)
note: template<class ContainerA> unsigned int Star::commonLength(ContainerA, unsigned int)

I'm assuming I'm specifying my template parameters wrong or something like that. I also attempted to restructure and eliminate that function altogether, but then I get the same error against getTupleFrom.

Can anyone explain to me why I'm stupid? Because I just don't know what I'm doing wrong. :(

解决方案

Well, this works:

#include <iostream>
#include <deque>
#include <tuple>
#include <string>
#include <type_traits>
#include <algorithm>
#include <limits>

template <class ContainerA>
unsigned commonLength(unsigned len, const ContainerA &first) {
  unsigned firstLen = first.size();
  if (len > firstLen) {
    len = firstLen;
  }
  return len;
}


template <class ContainerA, class... Containers>
unsigned commonLength(unsigned len, const ContainerA &first, const Containers&... rest) {
  unsigned firstLen = first.size();
  if (len > firstLen) {
    len = firstLen;
  }
  return commonLength(len, rest...);
}

template <template <typename...> class Container, typename TypeA>
std::tuple<TypeA> getTupleFrom(unsigned index, Container<TypeA> const& first) {
  return std::tuple<TypeA>(first[index]);
}

template <template <typename...> class Container, typename TypeA, typename... Types>
std::tuple<TypeA, Types...> getTupleFrom(unsigned index, Container<TypeA> const& first, Container<Types> const&... rest) {
  return std::tuple_cat(std::tuple<TypeA>(first[index]), getTupleFrom(index, rest...));
}

template <template <typename...> class Container, typename... Types>
Container<std::tuple<Types...>> zip(Container<Types> const&... args) {
  unsigned len = commonLength(std::numeric_limits<unsigned>::max(), args...);
  Container<std::tuple<Types...>> res;
  std::tuple<Types...> item;

  for (unsigned i=0; i<len; i++) {
    item = getTupleFrom(i, args...);
    res.push_back(item);
  }

  return res;
}

int main() {

  std::deque<int> test1 = {1, 2, 3, 4};
  std::deque<std::string> test2 = {"hihi", "jump", "queue"};
  std::deque<float> test3 = {0.2, 8.3, 7, 123, 2.3};
  for (auto i : zip(test1, test2, test3)) {
    std::cout << std::get<0>(i) << std::get<1>(i) << std::get<2>(i) << std::endl;
  }
  //expected output:
  //1hihi0.2
  //2jump8.3
  //3queue7
}

It outputs exactly what you expected. The problems were:

• You weren't using const& containers in commonLength, while zip's arguments where const references.
• The unsigned parameter in commonLength coudln't be deduced, so I moved it to the beginning
• You declared/defined functions in the wrong order(A required B, but A was defined before B), so I reordered them.

Apparently clang 3.1 fails to deduce the template arguments in zip, but g++ 4.6 gets them fine.

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