我无法理解为什么移动构造函数没有被调用，而移动赋值能够，而如果我在 X 行中使用移动函数，它曾经调用移动构造函数。谁能告诉调用移动构造函数的方式或语法是什么。
#include <iostream>
#include <cstring>
#include <algorithm>
#include <memory>
using namespace std;
class String
{
char *s;
int len;
public:
String():s(nullptr),len(0){ cout<<"Default "; }
String(char *p)
{
if(p)
{
len = strlen(p);
s = new char[len];
strcpy(s,p);
}
else
{
s = nullptr;
len = 0;
}
cout<<"Raw ";
}
String(String &p)
{
if(p.s)
{
len = strlen(p.s);
s = new char[len];
strcpy(s,p.s);
}
else
{
s = nullptr;
len = 0;
}
cout<<"Copy ";
}
String & operator = (const String & p)
{
if(this != &p)
{
delete []s;
s = nullptr;
len = 0;
if(p.len)
{
len = p.len;
s = new char[len];
strcpy(s,p.s);
}
}
cout<<"Assignment ";
return *this;
}
String( String && p):s(nullptr),len(0) // move constructor
{
len = p.len;
s = p.s;
p.s = nullptr;
p.len = 0;
cout<<"Move Copy ";
}
String & operator = (String && p) // move assignment
{
if(this != &p)
{
delete []s;
len = 0;
s = nullptr;
if(p.len)
{
len = p.len;
s = p.s;
p.s = nullptr;
p.len = 0;
}
}
cout<<"Move Assignment ";
return *this;
}
~String() { delete []s; cout<<"Destructor \n"; }
void show() { cout<<s<<endl; }
};
int main()
{
String s1("Something ");
String s2(s1);
s1.show();
s2.show();
String s4(String("Nothing ")); // Line X
s4.show();
String s5;
s5 = String(s2);
s5.show();
return 0;
}
输出：
Raw Copy Something Something Raw Nothing 默认复制 Move 赋值 析构函数 析构函数 析构函数 析构函数 析构函数