将variadic模板列表中的整数值分配给static const std :: array成员

Question

请考虑以下代码段：

template<unsigned... IDs>
class MyClass{
public:
    static const std::array<unsigned, sizeof...(IDs)> ids { IDs... };
    PinIDs() = default;
};

然后使用该类：

MyClass<1,5,7,9> myClass;

目标是使ids的大小为4，并且分别包含值：（1,5,7,9）。

这种类型的对象是否可能，或者我是否必须删除静态限定符？如果不是如何用静态限定符写这个。该对象需要是默认的可构造的。

---

EDIT:

我尝试了Apple Apple的第一个解决方案，在Win7上使用MS Visual Studio 2017 CE，我得到了这个编译错误：

 1>------ Build started: Project: PracticeMath, Configuration: Debug Win32 ------
1>stdafx.cpp
1>PracticeMath.cpp
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2988: unrecognizable template declaration/definition
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2143: syntax error: missing ';' before '<'
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2059: syntax error: '<'
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2039: 'ids': is not a member of '`global namespace''
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2143: syntax error: missing ';' before '{'
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2447: '{': missing function header (old-style formal list?)
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(38): error C2065: 'myId': undeclared identifier
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(39): error C2065: 'myId': undeclared identifier
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(40): error C2065: 'myId': undeclared identifier
1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(44): error C2065: 'c': undeclared identifier
1>Done building project "PracticeMath.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

完整的原始来源如下：

#include <iostream>
#include <array>

template<unsigned... IDs>
class PinIDs{
public:
    static const std::array<unsigned, sizeof...(IDs)> ids;

    PinIDs() = default;

    const unsigned& operator[]( unsigned idx ) const {
        return ids[idx];
    }
};

template<unsigned... IDs>
const std::array<unsigned, sizeof...(IDs)> PinIDs<IDs...>::ids { IDs... };

int main() {
    PinIDs<4, 17, 19> myId;

    std::cout << myId[0] << " ";
    std::cout << myId[1] << " ";
    std::cout << myId[2] << " ";

    std::cout << "\nPress any key and enter to quit." << std::endl;
    char c;
    std::cin >> c;

    return 0;
}

感谢StoryTeller提出的事实，当我尝试应用Apple Apple的1st method时，我意外地混淆了MyClass而不是我的解决方案中的类的实际名称 - 项目。一旦我纠正它确实编译，构建并按预期运行。

Answer 1

你可以试试这个

---

#include <array>

template<unsigned... IDs>
class MyClass{
public:
   static const std::array<unsigned, sizeof...(IDs)> ids;
   MyClass() = default;
};

template<unsigned... IDs>
const std::array<unsigned, sizeof...(IDs)> MyClass<IDs...>::ids {IDs...};

int main(){
   MyClass<1,5,7,9> myClass;
   return myClass.ids[0];
}

---

或使用constexpr / inline（都需要c ++ 17）

---

#include <array>
template<unsigned... IDs>
class MyClass{
public:
    //static constexpr std::array<unsigned, sizeof...(IDs)> ids{IDs...};//or this
    static inline const std::array<unsigned, sizeof...(IDs)> ids{IDs...};
    MyClass() = default;
};

int main(){
MyClass<1,5,7,9> myClass;
    return myClass.ids[0];
}

Answer 2

有关基础知识，请参阅@apple apple的答案。我将添加C ++ 17方法来实现它。这与你原来的尝试非常接近。只需在变量中添加一个inline说明符：

template<unsigned... IDs>
class MyClass{
public:
    static inline const std::array<unsigned, sizeof...(IDs)> ids{ { IDs... } };
    MyClass() = default;
};

现在声明可以兼作定义。哦，请注意大括号。 std::array需要初始化为汇总。所以{}的一对std::array，以及它所拥有的内部原始阵列的一对。

Answer 3

你为什么不试试online compiler, supporting c++17？

template<unsigned... IDs>
class MyClass{
public:
    static constexpr std::array<unsigned, sizeof...(IDs)> ids { IDs... };
    MyClass() = default;
};

工作良好。对于在编译时计算的变量，你不需要static const inline，只需使用static constexpr