C++靜態成員鏈接時 "unresolved external symbol " : static "

Why are classes with static data members getting linker errors?

Because static data members must be explicitly defined in exactly one compilation unit. If you didn't do this, you'll probably get an "undefined external" linker error. For example:

// Fred.h

class Fred {
public:
  ...
private:
  static int j_;   // Declares static data member Fred::j_
  ...
};

The linker will holler at you ("Fred::j_ is not defined") unless you define (as opposed to merely declare) Fred::j_ in (exactly) one of your source files:

// Fred.cpp

#include "Fred.h"

int Fred::j_ = some_expression_evaluating_to_an_int;

// Alternatively, if you wish to use the implicit 0 value for static ints:
// int Fred::j_;

The usual place to define static data members of class Fred is file Fred.cpp (or Fred.C or whatever source file extension you use).