Initialize static constant member variable in nested class

Question

Given codes in language C++

/* file xxx.hpp */

class A
{
};

class B
{
private:
    class C
    {
        static const A a;
    };
};

How can I initialize the static constant member variable A a in nested class C?

Answer 1

Add the following to exactly one .cpp :

const A B::C::a;

Note that the posted code was only a declaration : this is the actual definition and initialisation.

Answer 2

In the cpp file with code that is to be compiled you should add const AB::C::a = {}; . The initializer (be it an initializer list in {...} for POD classes or aggregate types or a single constant value for built-in types) is optional - if it's not specified, a default constructor will be called for a . In case of primitive types, it should be set to 0.

updated:

As David has greatly remarked below, some compilers issue warnings when no initializer is specified for a static member definition. If there are some data members in class A and no initializer during definition of a is specified, my g++ 4.6.3 compiler issues the following warning (that is by default is treated as error):

test.cpp:26:9: error: uninitialized const 'B::C::a' [-fpermissive]

test.cpp:6:7: note: 'const class A' has no user-provided default

constructor test.cpp:11:8: note: and the implicitly-defined constructor does not initialize 'int A::test'

Answer 3

Like you would a regular static member.

In your implementation file:

const A B::C::a;

Answer 4

静态成员变量的初始化始终在变量的定义中执行，该变量必须位于程序中的单个翻译单元中：

const A B::C::a = {};

Answer 5

在源文件（xxx.cpp）中，定义变量：

const A B::C::a;