Move assignment operator, move constructor

Question

I've been trying to nail down the rule of 5, but most of the information online is vastly over-complicated, and the example codes differ.

Even my textbook doesn't cover this topic very well.

On move semantics:

Templates, rvalues and lvalues aside, as I understand it, move semantics are simply this:

int other     = 0;           //Initial value
int number    = 3;           //Some data

int *pointer1 = &number;     //Source pointer
int *pointer2 = &other;      //Destination pointer

*pointer2     = *pointer1;   //Both pointers now point to same data 
 pointer1     =  nullptr;    //Pointer2 now points to nothing

//The reference to 'data' has been 'moved' from pointer1 to pointer2

As apposed to copying, which would be the equivalent of something like this:

pointer1      = &number;     //Reset pointer1

int newnumber = 0;           //New address for the data

newnumber     = *pointer1;   //Address is assigned value
pointer2      =  &newnumber; //Assign pointer to new address

//The data from pointer1 has been 'copied' to pointer2, at the address 'newnumber'

No explanation of rvalues, lvalues or templates is necessary, I would go as far as to say those topics are unrelated.

The fact that the first example is faster than the second, should be a given. And I would also point out that any efficient code prior to C++ 11 will do this.

To my understanding, the idea was to bundle all of this behavior in a neat little operator move() in std library.

When writing copy constructors and copy assignment operators, I simply do this:

Text::Text(const Text& copyfrom) {
    data  = nullptr;  //The object is empty
    *this = copyfrom;

}


const Text& Text::operator=(const Text& copyfrom) {
    if (this != &copyfrom) {
        filename = copyfrom.filename;
        entries  = copyfrom.entries;

        if (copyfrom.data != nullptr) {  //If the object is not empty
            delete[] data;
        }

        data = new std::string[entries];

        for (int i = 0; i < entries; i++) {
            data[i] = copyfrom.data[i];
            //std::cout << data[i];
        }
        std::cout << "Data is assigned" << std::endl;

    }

    return *this;
}

The equivalent, one would think, would be this:

Text::Text(Text&& movefrom){
    *this = movefrom;
}

Text&& Text::operator=(Text&& movefrom) {
    if (&movefrom != this) {
        filename = movefrom.filename;
        entries  = movefrom.entries;
        data     = movefrom.data;

        if (data != nullptr) {
            delete[] data;
        }

        movefrom.data    = nullptr;
        movefrom.entries = 0;
    }
    return std::move(*this);
}

I'm quite certain this won't work, so my question is: How do you achieve this type of constructor functionality with move semantics?

Answer 1

It's not entirely clear to me what is supposed to be proved by your code examples -- or what the focus is of this question is.

Is it conceptually what does the phrase 'move semantics' mean in C++?

Is it "how do I write move ctors and move assignment operators?" ?

Here is my attempt to introduce the concept. If you want to see code examples then look at any of the other SO questions that were linked in comments.

---

Intuitively, in C and C++ an object is supposed to represent a piece of data residing in memory. For any number of reasons, commonly you want to send that data somewhere else.

Often one can take a direct approach of simply passing a pointer / reference to the object to the place where the data is needed. Then, it can be read using the pointer. Taking the pointer and moving the pointer around is very cheap, so this is often very efficient. The chief drawback is that you have to ensure that the object will live for as long as is needed, or you get a dangling pointer / reference and a crash. Sometimes that's easy to ensure, sometimes its not.

When it isn't, one obvious alternative is to make a copy and pass it (pass-by-value) rather than passing by reference. When the place where the data is needed has its own personal copy of the data, it can ensure that the copy stays around as long as is needed. The chief drawback here is that you have to make a copy, which may be expensive if the object is big.

A third alternative is to move the object rather than copying it. When an object is moved, it is not duplicated, and instead becomes available exclusively in the new site, and no longer in the old site. You can only do this when you won't need it at the old site anymore, obviously, but in that case this saves you a copy which can be a big savings.

When the objects are simple, all of these concepts are fairly trivial to actually implement and get right. For instance, when you have a trivial object, that is, one with trivial construction / destruction, it is safe to copy it exactly as you do in the C programming language, using memcpy. memcpy produces a byte-for-byte copy of a block of bytes. If a trivial object was properly initialized, since its creation has no possible side-effects, and its later destruction doesn't either, then memcpy copy is also properly initialized and results in a valid object.

However, in modern C++ many of your objects are not trivial -- they may "own" references to heap memory, and manage this memory using RAII, which ties the lifetime of the object to the usage of some resource. For instance, if you have a std::string as a local variable in a function, the string is not totally a "contiguous" object and rather is connected to two different locations in memory. There is a small, fixed-size (sizeof(std::string), in fact) block on the stack, which contains a pointer and some other info, pointing to a dynamically sized buffer on the heap. Formally, only the small "control" part is the std::string object, but intuitively from the programmer's point the buffer is also "part" of the string and is the part that you usually think about. You can't copy a std::string object like this using memcpy -- think about what will happen if you have std::string s and you try to copy sizeof(std::string) bytes from address &s to get a second string. Instead of two distinct string objects, you'll end up with two control blocks, each pointing to the same buffer. And when the first one is destroyed, that buffer is deleted, so using the second one will cause a segfault, or when the second one is destroyed, you get a double delete.

Generally, copying nontrivial C++ objects with memcpy is illegal and causes undefined behavior. This is because it conflicts with one of the core ideas of C++ which is that object creation and destruction may have nontrivial consequences defined by the programmer using ctors and dtors. Object lifetimes may be used to create and enforce invariants which you use to reason about your program. memcpy is a "dumb" low-level way to just copy some bytes -- potentially it bypasses the mechanisms that enforce the invariants which make your program work, which is why it can cause undefined behavior if used incorrectly.

Instead, in C++ we have copy constructors which you can use to safely make copies of nontrivial objects. You should write these in a way that preserves what invariants you need for your object. The rule of three is a guideline about how to actually do that.

The C++11 "move semantics" idea is a collection of new core language features which were added to extend and refine the traditional copy construction mechanism from C++98. Specifically, it's about, how do we move potentially complex RAII objects, not just trivial objects, which we already were able to move. How do we make the language generate move constructors and such for us automatically when possible, similarly to how it does it for copy constructors. How do we make it use the move options when it can to save us time, without causing bugs in old code, or breaking core assumptions of the language. (This is why I would say that your code example with int's and int *'s has little to do with C++11 move semantics.)

The rule of five, then, is the corresponding extension of the rule of three which describes conditions when you may need to implement a move ctor / move assignment operator also for a given class and not rely on the default behavior of the language.