c++11: 委托(delegate)构造函数 - 无法选择构造函数模板？

Question

这是后续问题:
c++11 dedicated "proxy constructors" delegating to private univeral reference constructor?

我想去掉那里使用的“enum class Dummy”。

但我没有设法委托(delegate)给模板构造函数。
请参见下面的代码示例。

#include <iostream>
#include <string>
#include <typeinfo>

class MyClass
{

private:

    template <class T>
    MyClass(T&& data)
    : _data(std::forward<T>(data))
    {
        std::cout << "MyClass universal reference template c'tor" << std::endl;
    }

public:


    // proxy c'tors delegating to universal reference c'tor
    MyClass (std::string const & data)
    : MyClass<std::string>(data)
    {
        std::cout << "MyClass lvalue c'tor" << std::endl;
    }

    MyClass (std::string && data)
    : MyClass<std::string>(std::move(data))
    {
        std::cout << "MyClass rvalue c'tor" << std::endl;
    }

private:

    std::string _data;

};

int
main(
        int,
        char**)
{

    {
        std::string str("demo");
        MyClass myClass(str);
    }

    {
        MyClass myClass("hello, world");
    }

    return 0;
}

我收到以下错误:

main2.cpp: In constructor 'MyClass::MyClass(const string&)':
main2.cpp:21:7: error: 'class MyClass MyClass::MyClass' is not a non-static data member of 'MyClass'
 : MyClass<std::string>(data)
   ^
main2.cpp:21:14: error: expected '(' before '<' token
 : MyClass<std::string>(data)
          ^
main2.cpp:21:14: error: expected '{' before '<' token
main2.cpp: In constructor 'MyClass::MyClass(std::__cxx11::string&&)':
main2.cpp:27:7: error: 'class MyClass MyClass::MyClass' is not a non-static data member of 'MyClass'
 : MyClass<std::string>(std::move(data))
   ^
main2.cpp:27:14: error: expected '(' before '<' token
 : MyClass<std::string>(std::move(data))
          ^
main2.cpp:27:14: error: expected '{' before '<' token
main2.cpp: In function 'int main(int, char**)':
main2.cpp:11:5: error: 'MyClass::MyClass(T&&) [with T = std::__cxx11::basic_string<char>&]' is private
 MyClass(T&& data)
 ^
main2.cpp:46:28: error: within this context
     MyClass myClass(str);
                        ^
main2.cpp:11:5: error: 'MyClass::MyClass(T&&) [with T = const char (&)[13]]' is private
 MyClass(T&& data)
 ^
main2.cpp:50:39: error: within this context
     MyClass myClass("hello, world");

Answer 1

这并不特定于委派构造函数。构造函数(和转换函数)没有名称，因此在语言中无法为它们提供显式模板参数。引用 C++14，[temp.mem] 14.5.2/5:

[ Note: Because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates. —end note ]

注释不是规范性的，但此注释只是明确说明了第 14 章中规则的后续内容。