将可变参数模板列表中的整数值分配给静态常量 std::array 成员

Question

考虑以下代码片段：

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

然后使用class作为：

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

Objective 将具有大小为 4 的 ids，并分别包含值：(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，将如何使用静态限定符来编写它。该对象需要是默认可构造的。

编辑：

~~我尝试了 Apple Apple 的第一个解决方案和 MS Visual Studio 2017 CE on Win7 我收到此编译器错误：~~

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 ==========

像这样的完整原始来源：

#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，我不小心混淆了 MyClass 而不是我解决方案中 class 的实际名称- 项目。一旦我更正它确实按预期编译、构建和运行。

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

基础知识参考@apple apple的回答。我将添加 C++17 方法来完成它。这与您最初的尝试非常接近。只需向变量添加一个 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，注意牙套。 std::array 需要初始化为聚合。因此，一对 {} 用于 std::array，一对用于它保存的内部原始数组。

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;
};

工作正常。您不需要 static const inline 用于在编译时计算的变量，只需使用 static constexpr

将可变参数模板列表中的整数值分配给静态常量 std::array 成员

Assigning integer values from variadic template list to static const std::array member

c++

templates

static-members

variadic

stdarray