c一个缺失的vtable错误

架构i386的未定义符号：

"vtable for A",referenced from:
      A::A() in A.o
      A::~MissionController() in A.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

代码片段

.h文件：

class A: public B

  public:
     A();
    ~A();

};

.cpp文件..

A::A()   
{


}

A::~A()
{


}