可以在定义（.cpp文件）中初始化静态float constexpr成员

Question

I would like to initialize my private static member "pi" in the definition of the class in order to tidy my code better having the initializations in the .cpp files.

When I try this error shows up: "Declaration of constexpr static data member 'pi' requires an initializer"

I'm using CLion 2018.3.4 and C++ 11.

I tried to work around and the only solution is to initialize the member in the declaration.

Other answers on Stack Overflow provided me a better knowledge but didn't answer my question.

// .h file

class Shape {
public:
    virtual void getArea();

private:
    static constexpr float pi; // the error shows up here
};

// .cpp file

#include "Shape.h"

const float Shape::pi = 3.14; //here I don't exactly know why it does not require constexpr, but it's fine even with and without const 

// what I think this is equivalent to (in .h file)
static constexpr float pi = 3.14;

I was expecting this to work like I was assigning "3.14" in the declaration.

I'm not defining a constructor, in that case I know it wouldn't work because the c'tor is meant to initialize an instance of the class while a static member is supposed to be already initialized being a global element in the "Shape" namespace.

What I suppose it is happening is that the linker tries to initialize the member in the header because it has to be done in the pre-processing phase while the .cpp file is used later.

Answer 1

With static constexpr members you cannot leave off the initializer in the class definition. A constexpr variable must be initialized when declared because it can be used after it is declared in a constant expression. This is detailed in [class.static.data]/3 of the C++11 standard

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). 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. [ Note: In both these cases, the member may appear in constant expressions. — end note ] The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

^{emphasis mine}

So, with that, your code needs to be

// .h file
class Shape {
public:
    virtual void getArea();

private:
    static constexpr float pi = 3.14; // we initialize here so it can be used.
};

// .cpp file
constexpr float Shape::pi; // we define here so it can be odr-used

Do note that this has changed in C++17. With the introduction of inline variables static constexpr member variables no longer need to be defined outside of the class. The compiler will handle it for you and ensure only a single defentition of the object exists. You can still define the member if you want, but that ability is deprecated and will most likely be removed in a future standard revision. The new text for [class.static.data]/3 is

If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer. An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static.constexpr]). Declarations of other static data members shall not specify a brace-or-equal-initializer.

^{emphasis mine}

and [dcl.constexpr]/1 says that a static constexpr variable is implicitly inline

The constexpr specifier shall be applied only to the definition of a variable or variable template or the declaration of a function or function template. The consteval specifier shall be applied only to the declaration of a function or function template. A function or static data member declared with the constexpr or consteval specifier is implicitly an inline function or variable ([dcl.inline]) . If any declaration of a function or function template has a constexpr or consteval specifier, then all its declarations shall contain the same specifier.

^{emphasis mine}