声明没有定义的 static 成员

Question

class A declares static variable b , but there is NO definition of b in the code.

Result:

1. The code can run successfully.
2. There is no output. So the code do not call the constructor and destructor of b .

My question:

1. when class A declares b , how to find b's definition?
2. why the code can run successfully?

#include <iostream>
#include <vector>
#include <boost/shared_ptr.hpp>

class B
{
public:
    B()
    {
        std::cout << "go into B constructor" << std::endl;
    };


    B::~B()
    {
        std::cout << "go into B destructor" << std::endl;
    };


    void output()
    {
        std::cout << "B output" << std::endl;
    }
};

class A
{
public:
    static B b;
    int i;
};

//B A::b;
int main()
{
    A a;
//    A::b.output();
    return 0;
}

Answer 1

In this code A::b is not used at all, so no definition needed :

int main()
{
    A a;
    return 0;
}

In this code A::b is used, so definition of A::b is needed :

 int main()
 {
    A::b.output();
    return 0;
 }

Fot non-const static data member to be initialized, this has to be done explicitly, for example add BA::b after class A {... }:

class A
{
public:
    static B b; // declaration of A::b
    int i;
};

B A::b; // definition of A::b

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