Question

很抱歉，如果这是重复的话这是关于静态指针类成员我无法理解的一段示例代码。

#include <iostream>

class MyField {
 public:
  MyField() { std::cout << "ctr for MyField\n"; }
  ~MyField() { std::cout << "dtr for MyField\n"; }
  friend std::ostream &operator<<(std::ostream &os, MyField field) {
    os << "dump for MyField with addr: " << &field << std::endl;
    return os;
  }
};

class MyClass {
 private:
  static MyField field_;

 public:
  static MyField *p_field_;
  static void setField(MyField const &field) { field_ = field; }
  static void setPField(MyField *p_field) { p_field_ = p_field; }
  static void dumpField() { std::cerr << "field_: " << field_; }
  static void dumpPField() { std::cerr << "p_field_: " << *p_field_; }
  MyClass() { std::cout << "ctr for MyClass\n"; }
  ~MyClass() { std::cout << "dtr for MyClass\n"; }
};

int main(void) {
  MyClass::setPField(NULL);  /// undefined reference to MyClass::p_field_
  MyClass::p_field_ = NULL;  /// undefined reference to MyClass::p_field_
  MyField field;
  MyClass::setField(field);    /// fine
  MyClass::p_field_ = &field;  /// undefined reference to MyClass::p_field_
  MyClass::setPField(&field);  /// undefined reference to MyClass::p_field_
  MyClass();
  MyClass::dumpField();   /// fine
  MyClass::dumpPField();  /// fine
  return 0;
}

对于类MyClass，为什么Field field_（setField）的集合有效，但p_field_（setPField）无效？它仍可用于访问p_field_（dumpPField）的地址？

BTW，我试过gcc4.8 / clang3.4，有或没有--std=c++11。

Answer 1

添加：

MyField * MyClass::p_field_ = NULL;

在任何功能之外，例如就在int main() {

之前

在C ++中为静态指针类成员链接期间的未定义引用

1 个答案: