如何使用可变模板参数调用模板函数？

Question

下面是我的简单可变参数模板函数。此模板将 std::tuple 作为其输入参数之一。但它拒绝编译并出现错误 "template argument deduction/substitution failed".

任何人都可以指出我犯的错误吗？

#include <tuple>

using namespace std;

template<typename... TT, typename ReturnType>
ReturnType& getValue(int ind, std::tuple<TT...>& t, ReturnType& val) {                                                                                                                                                                                                                                                                                                                        
    return val;                                                                                                                                                                                                                                                                                                                                                                               
}

int main() {                                                                                                                                                                                                                                                                                                                                                                                  
    std::string str("Hello"), result;                                                                                                                                                                                                                                                                                                                                                         
    std::tuple<std::string> t = std::make_tuple(str);                                                                                                                                                                                                                                                                                                                                         

    getValue<std::tuple<std::string>, std::string>(0, t, result);                                                                                                                                                                                                                                                                                                                             
    return 0;                                                                                                                                                                                                                                                                                                                                                                                 
}

下面是编译输出。

g++ -c tuple.cc -std=c++1z; g++ -o tuple tuple.o
tuple.cc: In function ‘int main()’:
tuple.cc:15:64: error: no matching function for call to ‘getValue(int, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&, std::__cxx11::string&)’
     getValue<std::tuple<std::string>, std::string>(0, t, result);
                                                                ^
tuple.cc:7:13: note: candidate: template<class ... TT, class ReturnType> ReturnType& getValue(int, std::tuple<_Elements ...>&, ReturnType&)
 ReturnType& getValue(int ind, std::tuple<TT...>& t, ReturnType& val) {
             ^
tuple.cc:7:13: note:   template argument deduction/substitution failed:
tuple.cc:15:64: note:   mismatched types ‘std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’ and ‘std::__cxx11::basic_string<char>’
     getValue<std::tuple<std::string>, std::string>(0, t, result);
                                                                ^

谢谢。

Answer 1

getValue(0, t, result);   

那将编译。 ...TT 不是元组，它是 std::string.

你试图用 ...TT 来调用它 std::tuple<std::string>, std::string, ... 这当然不匹配 std::string.