### 移动构造函数和赋值运算符与复制省略

``````#include<iostream>
using namespace std;

int counter = 0; // to keep count of the created objects

class X {
private:
int id = 0; // hopefully unique identifyier
int n = 0;
int * p;
public:
// special member functions (ctor, dtor, ...)
X()           : id(counter++), n(0),   p(NULL)       { cout << "default ctor (id " << id << ")\n"; }
X(int n)      : id(counter++), n(n),   p(new int[n]) { cout << "param ctor (id " << id << ")\n"; };
X(const X& x) : id(counter++), n(x.n), p(new int[n]) {
cout << "copy ctor (id " << id << ") (allocating and copying " << n << " ints)\n";
for (int i = 0; i < n; ++i) {
p[i] = x.p[i];
}
};
X(X&& x)      : id(counter++), n(x.n), p(x.p) {
cout << "move ctor (id " << id << ")\n";
x.p = NULL;
x.n = 0;
};
X& operator=(const X& x) {
cout << "copy assignment (";
if (n < x.size() && n > 0) {
cout << "deleting, ";
delete [] p;
n = 0;
}
if (n == 0) {
cout << "allocating, and ";
p = new int[n];
}
n = x.size();
cout << "copying " << n << " values)";
for (int i = 0; i < n; ++i) {
p[i] = x.p[i];
}
cout << endl;
return *this;
};
X& operator=(X&& x) {
this->n = x.n;
this->p = x.p;
x.p = NULL;
x.n = 0;
cout << "move assignment (\"moving\" " << this->n << " values)\n";
return *this;
};
~X() {
cout << "dtor on id " << id << " (array of size " << n << ": " << *this << ")\n";
delete [] p;
n = 0;
}
// getters/setters
int size() const { return n; }

// operators
int& operator[](int i) const { return p[i]; };
X operator+(const X& x2) const {
cout << "operator+\n";
int n = min(x2.size(), this->size());
X t(n);
for (int i = 0; i < n; ++i) {
t.p[i] = this->p[i] + x2.p[i];
}
return t;
};

// friend function to slim down the cout lines
friend ostream& operator<<(ostream&, const X&);
};

int main() {
X x0;
X x1(5);
X x2(5);
x1[2] = 3;
x2[3] = 4;
cout << "\nx0 = x1 + x2;\n";
x0 = x1 + x2;
cout << "\nX x4(x1 + x2);\n";
X x4(x1 + x2);
cout << x4 << endl;
cout << '\n';
}

// function to slim down the cout lines
ostream& operator<<(ostream& os, const X& x) {
os << '[';
for (int i = 0; i < x.size() - 1; ++i) {
os << x.p[i] << ',';
}
if (x.size() > 0) {
os << x.p[x.size() - 1];
}
return os << ']';
}
``````

``````\$ clear && g++ moves.cpp && ./a.out
``````

``````default ctor (id 0)
param ctor (id 1)
param ctor (id 2)

x0 = x1 + x2;
operator+
param ctor (id 3)
move assignment ("moving" 5 values)
dtor on id 3 (array of size 0: [])

X x4(x1 + x2);
operator+
param ctor (id 4)
[0,0,3,4,0]

dtor on id 4 (array of size 5: [0,0,3,4,0])
dtor on id 2 (array of size 5: [0,0,0,4,0])
dtor on id 1 (array of size 5: [0,0,3,0,0])
dtor on id 0 (array of size 5: [0,0,3,4,0])
``````

``````clear && g++ -fno-elide-constructors moves.cpp && ./a.out
``````

``````default ctor (id 0)
param ctor (id 1)
param ctor (id 2)

x0 = x1 + x2;
operator+
param ctor (id 3)
move ctor (id 4)
dtor on id 3 (array of size 0: [])
move assignment ("moving" 5 values)
dtor on id 4 (array of size 0: [])

X x4(x1 + x2);
operator+
param ctor (id 5)
move ctor (id 6)
dtor on id 5 (array of size 0: [])
move ctor (id 7)
dtor on id 6 (array of size 0: [])
[0,0,3,4,0]

dtor on id 7 (array of size 5: [0,0,3,4,0])
dtor on id 2 (array of size 5: [0,0,0,4,0])
dtor on id 1 (array of size 5: [0,0,3,0,0])
dtor on id 0 (array of size 5: [0,0,3,4,0])
+enrico:CSGuild\$
``````

#### 1 个答案:

• 为什么没有为`X x4(x1 + x2)`调用move构造函数？
• 为什么在禁用复制省略时，move构造函数被调用了两次？

### 第二个问题

1. `X x4(x1 + x2)`范围，我们在其中构造`operator+`并将其返回；
2. 我们称为`X`的{​​{1}}范围；
3. 我们从`main`构建`X x4(x1 + x2)`的{​​{1}};

• 将结果从`X constructor`移至`X`（移至`x1 + x2`）；和
• `operator+`的内容移至`main`