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;

目标是让ids的大小为4，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？如果不是如何用静态限定符写这个。该对象需要是默认的可构造的。

<小时/> 修改

~~我在Win7上尝试了Apple Apple的第一个解决方案和MS Visual Studio 2017 CE 我收到了这个编译错误：~~

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 ==========

完整的原始资源如下：

#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; }

<击>

感谢StoryTeller提出的事实，当我尝试应用Apple Apple 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

请参阅@apple apple的基本答案。我只是添加了C ++ 17的方法来实现它。这与你原来的尝试非常接近。只需在变量中添加inline说明符：

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

现在宣言可以兼作定义。哦，请注意大括号。 std::array需要初始化为聚合。因此{}有一对std::array，而它持有的内部原始数组有一对。

Answer 3

为什么不用online compiler, supporting c++17尝试？

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

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

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

3 个答案: