Move semantics in C++11

阿新 • • 發佈：2018-12-23

After studying an example for Move Semantics of C++11, I write a more complete code snippet:

C++

#include <iostream>

using namespace std;

class Intvec {
  public:
    explicit Intvec(size_t num = 0)
      : m_size(num), m_data(new int[m_size]) {
      log("constructor");
    }

    ~Intvec() {
      log("destructor");
      if (m_data) {
        delete[] m_data;
        m_data = 0;
      }
    }

    Intvec(const Intvec& other) : m_size(other.m_size), m_data(new int[m_size]) {
      log("copy constructor");
      for (size_t i = 0; i < m_size; ++i)
        m_data[i] = other.m_data[i];
    }

    Intvec(Intvec&& other) {
      log("move constructor");
      std::swap(m_size, other.m_size);
      std::swap(m_data, other.m_data);
      other.m_size = 0;
      other.m_data = nullptr;
    }

    Intvec& operator=(const Intvec& other) {
      log("copy assignment operator");
      Intvec tmp(other);
      std::swap(m_size, tmp.m_size);
      std::swap(m_data, tmp.m_data);
      return *this;
    }

    Intvec& operator=(Intvec&& other) {
      log("move assignment operator");
      std::swap(m_size, other.m_size);
      std::swap(m_data, other.m_data);
      return *this;
    }

  private:
    void log(const char* msg) {
      cout << "[" << this << "] " << msg << "\n";
    }

    size_t m_size;
    int* m_data;
};

int main(void) {
  Intvec v1(20);
  Intvec v2;

  cout << "assigning lvalue...\n";
  v2 = v1;
  cout << "ended assigning lvalue...\n";

  //Intvec v4 = std::move(Intvec(30));
  Intvec v3;
  cout << "move assigning...\n";
  v3 = Intvec(30);
  cout << "ended move assigning...\n";

  cout << "move constructor\n";
  Intvec v4 = std::move(v3);
  cout << "ended move constructor\n";

  return 0;
}

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#include <iostream>usingnamespacestd;classIntvec{public:explicitIntvec(size_t num=0):m_size(num),m_data(newint[m_size

]){log("constructor");}~Intvec(){log("destructor");if(m_data){delete[]m_data;m_data=0;}}Intvec(constIntvec&other):m_size(other.m_size),m_data(newint[m_size]){log("copy constructor");for(size_ti=0;i<m_size;++i)m_data[i]=other.m_data[i];}Intvec(Intvec&&other){log("move constructor");std::

swap(m_size,other.m_size);std::swap(m_data,other.m_data);other.m_size=0;other.m_data=nullptr;}Intvec&operator=(constIntvec&other){log("copy assignment operator");Intvec tmp(other);std::swap(m_size,tmp.m_size);std::swap(m_data,tmp.m_data);return*this;}Intvec&operator=(Intvec&&other){log("move assignment operator");std::swap(m_size,other.m_size);std::swap(m_data,other.m_data);return*this;}private:voidlog(constchar*msg){cout<<"["<<this<<"] "<<msg<<"\n";}size_t m_size;int*m_data;};intmain(void){Intvec v1(20);Intvec v2;cout<<"assigning lvalue...\n";v2=v1;cout<<"ended assigning lvalue...\n";//Intvec v4 = std::move(Intvec(30));Intvec v3;cout<<"move assigning...\n";v3=Intvec(30);cout<<"ended move assigning...\n";cout<<"move constructor\n";Intvec v4=std::move(v3);cout<<"ended move constructor\n";return0;}

Pay attention to last two lines in ‘move constructor’:

C++

...
      other.m_size = 0;
      other.m_data = nullptr;
...

1234	...other.m_size=0;other.m_data=nullptr;...

Since ‘move constructor’ will not set initial value for m_size and m_data of ‘v4’, the m_size and m_data of ‘v3’ will be uninitial after swaps. Adding the two lines of code means to initialize m_size and m_data of ‘v3’.

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