CODEWITHSUNDEEP
c++static-methodsstatic-memberslifetime
Hongxu Chen
Hongxu Chen

Reputation: 5350

undefined reference during linkage for a static pointer class member in C++

Sorry if this is a duplicate.
Here's one piece of example code that I cannot understand about static pointer class member.

#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;
}

For class MyClass, why the set for Field field_(setField) works but not for p_field_(setPField)? And it still works for the access to address of p_field_(dumpPField)?

BTW, I tried gcc4.8/clang3.4 with or without --std=c++11.

Upvotes: 1

Views: 1947

Answers (1)

Zyend
Zyend

Reputation: 602

Add:

MyField * MyClass::p_field_ = NULL;

outside of any function, e.g. just before int main() {

Upvotes: 4

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