Is it possible to define a 'move-and-swap idiom' equivalent in C++03

问题

I'm bound to C++03 and I have a non-copyable object (e.g. holding a resource).

I need to use move-and-swap semantics to be able to do something similar and avoid copies:

MyClass returnMyClass(void)
{
    MyClass temp;
    // fill 'temp' members with actual data
    return temp;
}

int main(void)
{
    MyClass test;
    test = returnMyClass(); // need to avoid copies
}

Is it possible to respect all these requirements in C++03?

It is basically the same case of this, but for C++03.

To put the question in other words:

Given a non-copyable class MyClass, is it somehow possible in C++03 to do MyClass test = returnMyClass();?

I'm afraid the answer is simply no, but maybe I'm missing some trick.

回答1:

There is no magic in move semantics. It's just another overload. The rvalue reference thing is a nice convenience that is not really essential.

template <class T>
struct rref { 
    rref (T& t) : t(t) {}
    T& t; 
};

template<class T>
rref<T> move(const T& t) {
   return rref<T>(const_cast<T&>(t));
}

// you now can do a "move ctor"
class Foo {
   Foo(rref<Foo>) { ... }
};

Now you also need NRVO to kick in for this to work. It is not guaranteed by the standard, but it is provided by pretty much every implementation. To ensure it really happens, you may declare but not define a copy ctor.

Full working demo

回答2:

If you can initialize your object test at declaration you will be able to avoid copying because copy elision will be performed.

class MyClass
{
  public:
    MyClass(){std::cout << "Default Constructor" << std::endl;}
    MyClass(const MyClass &){std::cout << "CPYConstructor" << std::endl;}
    ~MyClass() {std::cout << "Destructor" << std::endl;}

};


MyClass returnMyClass(void)
{
    MyClass temp;
    // fill 'temp' members with actual data
    return temp;
}

int main(void)
{
    MyClass test = returnMyClass(); // need to avoid copies
}

Output:

Default Constructor
Destructor

来源：https://stackoverflow.com/questions/60296661/is-it-possible-to-define-a-move-and-swap-idiom-equivalent-in-c03

标签

c++

move

c++03