C++编译优化备忘

基于GCC测试：http://www.tutorialspoint.com/compile_cpp11_online.php

const A& a=fun() 与 A a= fun()

1、方法fun返回值为 const A& 时，前者比后者 减少一次拷贝构造

#include <iostream>

using namespace std;

    class  A
    {
  
    public:

        A() {   
        cout << "--默认构造函数--" << endl; 
 }

        A(const A& a) {  cout << "--拷贝构造函数--" << endl; }

        A & operator =(const A &a)
        {
            cout << "--赋值--" << endl;
            return *this;
        }
    };

class Test
{
 public:
      const A& GetA(){ cout <<&m_a << endl; return m_a;}
      
 private:
    A m_a;
};

int main()
{
 Test t;      
 A vec;
   cout << "------" << endl; 
   A a=t.GetA();
   cout <<&a << endl;
  cout << "------" << endl; 
   const A& b=t.GetA();
   cout <<&b << endl;
   return 0;
}

2、方法fun返回值为 A 时，相同。只会生成一个临时变量

 A GetA(){ cout <<&m_a << endl; return m_a;}

　　

 A GetA(){ A a;cout <<&a << endl; return a;}

　　

未实现move构造的类：A a=std::move(fun()) 与  A a=fun()

1、方法fun返回值为 const A& 时，相同，都调用一次拷贝构造

class Test
{
 public:
      const A& GetA(){ cout <<&m_a << endl;  return m_a;}
      
 private:
    A m_a;
};

int main()
{
   Test t;      
   cout << "------" << endl; 
   A a=t.GetA();
   cout <<&a << endl;
  cout << "------" << endl; 
   A b=std::move(t.GetA());
   cout <<&b << endl;
   return 0;
}

2、方法fun返回值为 A，前者比后者多一次拷贝构造

class Test
{
 public:
      A GetA(){ cout <<&m_a << endl;  return m_a;}
      
 private:
    A m_a;
};

int main()
{
   Test t;      
   cout << "------" << endl; 
   A a=t.GetA();
   cout <<&a << endl;
  cout << "------" << endl; 
   A b=std::move(t.GetA());
   cout <<&b << endl;
   return 0;
}

未实现move构造的类：A a;a=std::move(fun()) 与  A a;a=fun()

 与fun的返回值无关，均相同

class Test
{
 public:
      A GetA(){ cout <<&m_a << endl;  return m_a;}
      
 private:
    A m_a;
};

int main()
{
   Test t;      
   cout << "------" << endl; 
   A a;
   a=t.GetA();
   cout <<&a << endl;
  cout << "------" << endl; 
   A b;
   b=std::move(t.GetA());
   cout <<&b << endl;
   return 0;
}

 const A& GetA(){ cout <<&m_a << endl;  return m_a;}