C ++类可以包含一个静态const std :: array在头文件中内联初始化吗？ [英] Can a C++ class contain a static const std::array initialized inline in a header file?

问题描述

这是我得到的：

struct Foo
{
   static std::array<double, 4> acgt_default_background_frequencies() { return {0.281774, 0.222020, 0.228876, 0.267330}; }
};

但我更喜欢不使用函数一个变量，如下：

But I'd prefer to not use a function and instead just have a variable, like this:

struct Foo
{
   static constexpr std::array<double, 4> acgt_default_background_frequencies = {0.281774, 0.222020, 0.228876, 0.267330};
};

我想要编译，但是当我尝试使用 Foo :: acgt_default_background_frequencies 它会给链接器错误未定义的引用`Foo :: acgt_default_background_frequencies'。

What I want compiles, but when I try to use Foo::acgt_default_background_frequencies it gives the linker error "undefined reference to `Foo::acgt_default_background_frequencies'".

我想做的是什么？我认为读者对我的头部更清楚，如果我有值内联作为一个常量比隐藏它在.cpp文件，并有一个常数，而不是一个函数也看起来更清楚。是不是constexpr的点允许这样的东西？如果不可能，为什么不？

Is what I am trying to do possible? I think it is clearer to the reader of my header if I have the value inlined as a const than to hide it in the .cpp file and having a constant as opposed to a function also seems clearer. Isn't the point of constexpr to allow stuff like this? If it isn't possible, why not?

推荐答案

em>一个 static 数据成员，它有一个初始化器，但是你没有在任何地方提供定义。如果您 odr-use

What you have in the second example is a declaration of a static data member which has an initializer, but you haven't provided a definition anywhere. If you make odr-use of that member, a definition will be required.

要提供定义，请将以下内容添加到.cpp文件中。

To provide a definition, add the following to your .cpp file

constexpr std::array<double, 4> Foo::acgt_default_background_frequencies;

问题的声明在C ++ 14中有效，但请注意，在C ++ 11中需要一组额外的花括号，例如

The declaration in the question works in C++14, but note that in C++11 you need an extra set of curly braces, e.g.

struct Foo
{
  static constexpr std::array<double, AlphabetSize> acgt_default_background_frequencies = {{0.281774, 0.222020, 0.228876, 0.267330}};
};

相关标准从N3337 §9.4.2/ 3 [class.static.data]

The relevant standardese from N3337 §9.4.2/3 [class.static.data]

... A static 文字类型的资料成员在类定义中使用 constexpr 说明符声明;如果是，其声明应指定括号或初始值，其中每个 initializer-clause 是常数表达式。 ...如果在程序中使用odr（3.2），并且命名空间范围定义不包含初始化器，则成员仍然在命名空间范围中定义。

... A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. ... The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

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