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  1. // http://stackoverflow.com/questions/19826045/trying-to-write-move-constructor-in-terms-of-move-assignment/19846215#19846215
  2. // https://i...content-available-to-author-only...e.com/AnwDvD
  3. #include<algorithm>
  4. #include<cassert>
  5. #include<cstring>
  6. #include<iostream>
  7.  
  8. class String
  9. {
  10.     char*   m_data;
  11.     int     m_len;
  12.     public:
  13.     const volatile String&& operator=(String&) volatile const  = delete; // make it as undesirable as possible!
  14.     void swap(String& rhs) throw()
  15.     {
  16.         std::cout << __PRETTY_FUNCTION__ << std::endl;
  17.         std::swap(m_data, rhs.m_data);
  18.         std::swap(m_len,  rhs.m_len);
  19.     }
  20.     template<typename T>
  21.     String& operator=(T templated_copy_and_swap) {
  22.          std::cout << __PRETTY_FUNCTION__ << "  (Templated  ) Assignment by (optional) copy and (not-optional) swap" << std::endl;
  23.          templated_copy_and_swap.swap(*this);
  24.          return *this;
  25.     }
  26.     String(String&& move_constructor) throw()  // The move constructor, we wish to use operator= here
  27.         : m_data(NULL)
  28.         , m_len(0)
  29.     {
  30.         std::cout << __PRETTY_FUNCTION__ << std::endl;
  31.         //move_constructor.swap(*this); // This was the original line we wish to replace
  32.        operator=(std::move(move_constructor)); // It works!
  33.     }
  34.     String& operator=(String && move_assignment) {
  35.          std::cout << __PRETTY_FUNCTION__ << std::endl;
  36.          move_assignment.swap(*this);
  37.          return *this;
  38.      }
  39.  
  40.      /* Below are just some other constructors that are required
  41.       * to build a working demonstration
  42.       */
  43.      String(const String &copy_constructor) {
  44.          std::cout << __PRETTY_FUNCTION__ << "     Copy constructor" << std::endl;
  45.          m_len = copy_constructor.m_len;
  46.          m_data = new char[m_len+1];
  47.          strncpy(m_data, copy_constructor.m_data, m_len+1);
  48.     }
  49.     ~String() {
  50.          std::cout << __PRETTY_FUNCTION__ << std::endl;
  51.         if(m_data) {
  52.             std::cout << " ~\"" << m_data << "\"" << std::endl;
  53.             delete[] m_data;
  54.         } // otherwise, this String was moved-from. Nothing to delete
  55.     }
  56.     template<size_t lenPlusOne>
  57.     explicit String(const char (&x)[lenPlusOne]) {
  58.          std::cout << __PRETTY_FUNCTION__ << std::endl;
  59.          m_data = new char[lenPlusOne];
  60.          strncpy(m_data, x, lenPlusOne);
  61.          m_len = lenPlusOne - 1; // to deal with the trailing '\0'
  62.          assert(m_data[m_len] == '\0');
  63.     }
  64. };
  65.  
  66.  
  67. String returnsT() {
  68.     std::cout << __PRETTY_FUNCTION__ << "    Inside returnsT().  It will do some stuff here." << std::endl;
  69.     String s("returnsT");
  70.     std::cout << __PRETTY_FUNCTION__ << "    Inside returnsT().  About to return." << std::endl;
  71.     return s;
  72. }
  73.  
  74. int main() {
  75.     String s1("first");
  76.     String s2("second");
  77.     String s3("third");
  78.  
  79.     std::cout << std::endl;
  80.     std::cout << " === Situation A): Assign from l-value. Copy required." << std::endl << std::endl;
  81.     s1 = s2;
  82.  
  83.     {
  84.     std::cout << std::endl;
  85.     std::cout << " === Situation B): Assign from returnsT() returning T.  C++03 can optimize this, no copy *OR* move required after the return." << std::endl << std::endl;
  86.     s1 = returnsT();
  87.     }
  88.  
  89.     {
  90.     std::cout << std::endl;
  91.     std::cout << " === Situation C): Assign from move() returning T&&." << std::endl << std::endl;
  92.     s1 =             std::move(s3) ;
  93.     //s1 = StringRVRef(std::move(s3));
  94.     }
  95.  
  96.     std::cout << std::endl;
  97.     std::cout << " === Done. main() will finish now, destructing the three local strings." << std::endl << std::endl;
  98. }
  99.  
Success #stdin #stdout 0s 3476KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
String::String(const char (&)[lenPlusOne]) [with unsigned int lenPlusOne = 6u]
String::String(const char (&)[lenPlusOne]) [with unsigned int lenPlusOne = 7u]
String::String(const char (&)[lenPlusOne]) [with unsigned int lenPlusOne = 6u]

 === Situation A): Assign from l-value. Copy required.

String::String(const String&)     Copy constructor
String& String::operator=(T) [with T = String]  (Templated  ) Assignment by (optional) copy and (not-optional) swap
void String::swap(String&)
String::~String()
 ~"first"

 === Situation B): Assign from returnsT() returning T.  C++03 can optimize this, no copy *OR* move required after the return.

String returnsT()    Inside returnsT().  It will do some stuff here.
String::String(const char (&)[lenPlusOne]) [with unsigned int lenPlusOne = 9u]
String returnsT()    Inside returnsT().  About to return.
String& String::operator=(String&&)
void String::swap(String&)
String::~String()
 ~"second"

 === Situation C): Assign from move() returning T&&.

String& String::operator=(String&&)
void String::swap(String&)

 === Done. main() will finish now, destructing the three local strings.

String::~String()
 ~"returnsT"
String::~String()
 ~"second"
String::~String()
 ~"third"
https://ideone.com/lmFlYI
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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