C++ 如何初始化类型特征的静态 std::unordered_map？
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How to initialize a static std::unordered_map of a type trait?
提问by danijar
Given the following type trait, how can I initialize Fieldswith some std::pairs?
鉴于以下类型特征，我如何Fields用一些std::pairs 进行初始化？
template <>
struct ManagerDataTrait<Person>
{
static const std::unordered_map<std::string, std::string> Fields;
// ...
};
I tried using a lambda but Visual Studio says that Fieldsis not an entity that can be explicitly specialized.
我尝试使用 lambda，但 Visual Studio 表示这Fields不是可以显式专门化的实体。
template <>
const std::unordered_map<std::string, std::string> ManagerDataTrait<Person>::Fields = []{
std::unordered_map<std::string, std::string> fields;
fields.insert(std::make_pair("height", "FLOAT"));
fields.insert(std::make_pair("mass", "FLOAT"));
return fields;
};
If there is no way to use static members like this in traits, which alternatives do I have to store the information in a trait? (Fieldsholds a SQL database structure.)
如果无法在特征中使用这样的静态成员，那么我必须将信息存储在特征中的哪些替代方法？（Fields包含 SQL 数据库结构。）
Update: The member might be also constbut that shouldn't be the point.
更新：该成员可能也是，const但这不应该是重点。
采纳答案by Jonathan Wakely
Kerrek SB's answer would be the right answer in general:
Kerrek SB 的答案通常是正确的答案：
const std::unordered_map<std::string, std::string> ManagerDataTrait<Person>::Fields{
{ "blah", "blah" }
// ...
};
(N.B. no template<>because you're defining a member of a specialization, not a specialization)
（注意不，template<>因为您正在定义专业化的成员，而不是专业化）
But that isn't supported by Visual C++, so the other alternative is to initialize the map with a function call, and return a map with the desired contents from the function:
但这不受 Visual C++ 支持，因此另一种选择是使用函数调用初始化映射，并从函数返回具有所需内容的映射：
std::unordered_map<std::string, std::string>
getFields()
{
std::unordered_map<std::string, std::string> fields;
fields["blah"] = "blah";
// ...
return fields;
}
const std::unordered_map<std::string, std::string> ManagerDataTrait<Person>::Fields = getFields();
A lambda is just syntactic sugar for doing the same thing, and I'm not sure it's clearer to use a lambda because the syntax is a bit uglier.
lambda 只是做同样事情的语法糖，我不确定使用 lambda 是否更清晰，因为语法有点难看。
回答by Kerrek SB
You realize you can initialize maps from braced lists?
你意识到你可以从花括号列表初始化地图吗？
std::unordered_map<std::string, std::string> m { { "a", "bc" }
, { "b", "xy" }
// ...
};
回答by NiRuKa
Visual C++ now supports static initialization from braced lists, so you could do something like this:
Visual C++ 现在支持从花括号列表进行静态初始化，因此您可以执行以下操作：
const std::unordered_map<std::string, std::string> ManagerDataTrait<Person>::Fields{ { "abc", "xyz" }, { "cde", "zyx" } ...};
