移动语意及非移动语意下的函数返回过程分析

    c++11的移动语意特性带来了很多好处，其中之一，就是让参数的“传递”更加自然，参数到了这里就真的像是“传”进了方法。应用在构造函数上，很大程度上提升了对象构造的效率。这点在大对象时的体现尤其明显。与此同时，编译器的RVO（return value optimization ），也给方法返回值提供了类似移动语意的编译器级别优化。为了更好的理解这个过程，写了两段示例代码，分别使用了移动构造函数和拷贝构造函数，来分析对象的传递和构造过程。

代码1，拷贝构造函数

/**
 * compile cmd: g++ -g -o test a.cc -std=c++0x -fno-elide-constructors
 */
#include <iostream>

class my
{
public:
    int a {1};

public:
    my()
    {
        std::cout << "ctor_@" << this << std::endl;
    }
    my(const my& m)
    {
        std::cout << "cpy_ctor_src@" << &m << ";rcv@" << this << std::endl;
        a = m.a;
    }
    ~my()
    {
        std::cout << "dtor_@" << this << std::endl;
    }
    my& operator = (const my& m)
    {
        std::cout << "operator=_src@" << &m << ";rcv@" << this << std::endl;
        a = m.a;
        return *this;
    }
};

my get_my()
{
    my _m = my();
    return _m;
}

/**
 * 可能的输出，及分析如下
 * ctor_@0x7fff2bd494c0                             _n
 * ctor_@0x7fff2bd49490                             get_my()->my()临时变量
 * cpy_ctor_src@0x7fff2bd49490;rcv@0x7fff2bd49480   get_my()->_m 函数局部变量
 * dtor_@0x7fff2bd49490
 * cpy_ctor_src@0x7fff2bd49480;rcv@0x7fff2bd494f0   get_my()返回值临时变量
 * dtor_@0x7fff2bd49480
 * operator=_src@0x7fff2bd494f0;rcv@0x7fff2bd494c0  get_my()返回值临时变量->_n
 * dtor_@0x7fff2bd494f0
 * dtor_@0x7fff2bd494c0                             _n
 */
int main()
{
    my _n;
    _n = get_my();
    return 0;
}

代码2，移动构造函数

/**
 * compile cmd: g++ -g -o test moveable.cc -std=c++0x -fno-elide-constructors
*/
#include <iostream>

class HugeMem
{
public:
    int* c;
    int sz;

public:
    HugeMem(int size): sz(size > 0? size: 1)
    {
        c = new int[sz];
    }

    HugeMem(HugeMem&& hm)
    {
        c = hm.c;
        sz = hm.sz;
        hm.c = nullptr;
    }

    ~HugeMem()
    {
        delete [] c;
    }
};

class Moveable
{
public:
    int* i;
    HugeMem h;

public:
    Moveable(): i(new int(3)), h(1024)
    {
        std::cout << "Moveable_nor_ctor_called_src@" << this << std::endl;
    }

    Moveable(Moveable&& m): i(m.i), h(std::move(m.h))
    {
        std::cout << "Moveable_mov_ctor_called_src@" << &m << ";rcv@" << this << std::endl;
        m.i = nullptr;
    }

    ~Moveable()
    {
        std::cout << "Moveable_dtor_called_src@" << this << std::endl;
        delete [] i;
    }
};

#define output_moveable_addr(ma) 
{ 
    std::cout << std::hex << "huge_mem_from:" << __func__ << " @" << ma.h.c << std::endl; 
}

Moveable get_temp()
{
    Moveable tmp = Moveable();
    output_moveable_addr(tmp);

    return tmp;
}

int main()
{
    /**
     * 一种可能的输出及结果分析
     *
     * Moveable_nor_ctor_called_src@0x7ffffecc3eb0                      get_temp()->Moveable() 临时变量
     * Moveable_mov_ctor_called_src@0x7ffffecc3eb0;rcv@0x7ffffecc3e90   get_temp()->临时变量 => get_temp()->tmp
     * Moveable_dtor_called_src@0x7ffffecc3eb0                          get_temp()->Moveable() 临时变量销毁
     * huge_mem_from:get_temp @0x602030
     * Moveable_mov_ctor_called_src@0x7ffffecc3e90;rcv@0x7ffffecc3f10   get_temp()->tmp => get_temp()返回值临时变量
     * Moveable_dtor_called_src@0x7ffffecc3e90                          get_temp()->tmp销毁
     * Moveable_mov_ctor_called_src@0x7ffffecc3f10;rcv@0x7ffffecc3ef0   get_temp()返回值临时变量 => main()->a
     * Moveable_dtor_called_src@0x7ffffecc3f10                          get_temp()返回值临时变量销毁
     * huge_mem_from:main @0x602030
     * Moveable_dtor_called_src@0x7ffffecc3ef0                          main()->a 销毁
     */
    Moveable a(get_temp());
    output_moveable_addr(a);

    return 0;
}

#undef output_moveable_addr

完。