Move Semantics in C++11, Part 2: Design and Implementation of Special Move Functions

Jul 9, 2012

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Move semantics in C++11 can improve your code's performance dramatically. Danny Kalev concludes this series with a close look at the design and implementation of move-enabled code. Find out what the state of a moved-from object should be, and learn the guidelines for writing move constructors and move assignment operators.

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Part 1 of this series presented the semantics of C++11 move operations and their effects on the source and target. This article concludes this series by focusing on the formal properties of moving, providing guidelines for designing move functions, and illustrating the standard C++ move features.

The State of Moved-from Variables

According to the C++11 standard, the state of a moved-from object is unspecified, albeit valid. This loose definition gives implementers enough leeway to do all sorts of things when moving. For example, a moved-from object might be left in an "empty" state, similar to a default constructed object. Another option is to leave the moved-from object in its pre-move state. The policy you should employ depends on the properties of the type and its usage. Let's look at moved-from objects that retain their original value.

When moving from fundamental types such asintandfloat, the source variable may retain its original value:

int m=7, n=0; n=m; //"move" m to n.

Seemingly, the programmer should setmto zero because a moved-from object is usually left in a default-initialized state. However, we can think of good reasons whym应该留下完整:

In many cases, a moved-from object is discarded immediately after the move operation. If it has no destructor, it's more efficient to leave the moved-from object intact.
The C++11 requirement that a moved-from object has an unspecified state is satisfied even ifmremains intact.

You therefore need to zeromonly if you have a reason to do so. What might be a good reason for zeroing a variable after a move operation? Suppose you have two pointersp1andp2and you want to movep1top2. Leavingp1intact could be disastrous because destructors and cleanup code assume that a non-null pointer value indicates resource ownership. This approach could lead to the infamous aliasing problem where two pointers hold the same address. To avoid aliasing, always set a moved-from pointer tonullptr:

int *p1= new int(5); int *p2=nullptr; //move p1 to p2 p2=p1; p1=nullptr; delete p2; //ok delete p1; //also ok; deleting a null pointer is harmless

Moving Class Objects

If a variable represents resource ownership, you should set it to its default value explicitly, even if it's not a pointer. Consider:

class String { size_t len; char * buf; bool empty; //.. };

Question: Which of the three data members in this example (len,buf, andempty) should be set to zero in a moved-fromString? Answer: All three of them. Obviously,bufshould be set tonullptrto avoid aliasing. Certain applications may assume that iflenisn't zero or ifemptyisn'ttruethen theStringobject still owns resources. Therefore,everyvalue that indicates resource ownership should be set to zero—not just pointers. If you're not sure about the value of a data member, use the default constructor as your guide: The state of a moved-from object should be that of a default-initialized object.

Special Member Functions

The move constructors and move assignment operator are collectively known asmove special member functions. They present a dilemma: Which of the two, if any, should your class define?

Under certain conditions, C++隐式声明the special member functions in a class that doesn't declare them explicitly. In C++11, this rule also applies to the two move special member functions. Thus, if the definition of classCdoesn't explicitly declare a move assignment operator, one will be implicitly declared asdefaultedonly ifallof the following conditions are met:

Cdoesn't have a user-declared copy constructor.
Cdoesn't have a user-declared move constructor.
Cdoesn't have a user-declared copy assignment operator.
Cdoesn't have a user-declared destructor.
The move assignment operator would not be implicitly defined asdeleted.

Consider the following class:

struct S { int n; S& operator=(const S&) = default; };

C++ will not implicitly declare a move assignment or a move constructor for classCin this case becauseChas a user-declared assignment operator. By contrast, classTin the following example will have both an implicitly declared move assignment operator and an implicitly declared move constructor:

class T { int n; public: T(); };

What does an implicitly defined move special member function do? This issue was under debate for a long time. However, the final resolution says that an implicitly defined move assignment operator performs a member-wise move, typically by callingstd::move()(a new standard function that I'll discuss shortly).

C++11 Move Facilities

C++11 introduced several new facilities for moving. One of them is thestatic_castexpression:

static_cast(t);

Thestatic_castexpression converts thelvaluetto anrvalue referenceof typeT&&. This way, you can select which overloaded version of a function will be selected:f(T&&)orf(T&).

A class can define both move special member functions and copy special member functions. The designer of classStringtherefore may choose to define all six special member functions without risking ambiguity. The move special member function will be invoked when its argument is, or is convertible toString&&. Otherwise, the copy special member functions will be invoked.

Let's look at a concrete example of a legacy class that defines all four of the C++03 special member functions and that needs a C++11 facelift:

class String { public: // copy constructor String(const String& other) :len(other.len), buf(new char[other.len]) { std::cout << "In String's copy constructor. length = " << len << "." << std::endl; std::copy(other.buf, other.buf+len, buf); } // copy assignment operator String& operator=(const String& other) { std::cout << "In String's copy assignment operator. length = " <0) buf= new char [len]; else buf=nullptr; } ~String() { std::cout << "In String's destructor. length was" << len << "." << std::endl; delete[] buf; //even if buff is 0, this delete[] is safe len=0; } private: size_t len; char* buf; };

When implementing move special member functions, adhere to the following guideline:

The target object must release its resources (if any) before acquiring new resources.
Acquire the resources.
Set the source's data members to their default values.

Here's howString's move constructor looks like (thecoutstatements will enable you to see which special member function is called):

String::String(String&& other) //the canonical signature of a move constructor //first, set the target members to their default values : buf(nullptr) , len(0) { std::cout << "In String's move constructor. length = " <
       In a similar vein, the move assignment operator looks like this:
       字符串的字符串::操作符=(字符串& &其他)/ / canonical signature of a move assignment operator { std::cout << "In String's move assignment operator. length = " <
       Overloading with rvalue References
       The C++11 overloading rules recognize rvalue references. In addition, many Standard Library components now define overloaded sets of functions that take eitherT&orT&&as their parameters. If the argument is an rvalue, theT&&overloaded function will be selected. Otherwise, theconst T&overloaded function will be selected:
       #include  using namespace std; int main() { vector vs; vs.push_back(String(25)); //the string's size lets you track specific objects v.push_back(String(50)); // insert a new element into the second position v.insert(v.begin() + 1, String(60)); }
       When executing the program above, a C++11-compliant implementation will invoke only the move special member functions ofStringbecause in all three function calls the argument is an rvalue. More specifically it's anxvalue; that is, a temporary. (Thexis a hint that the object is about to expire shortly after its creation.) xvalues bind to rvalue references. However, if you use an lvalue argument,push_back(const T&)will be selected instead:
       vector vs; String s(15); vs.push_back(s); //uses String's copy constructor
       Even with lvalue arguments, you can force the implementation to select the move special member functions. For this purpose, either cast the lvalue toT&&usingstatic_cast, or use the standard functionstd::move()to do the same thing with fewer keystrokes:
       vector vs; String s1(15), s2(20); vs.push_back(static_cast(s1)); //uses String's move constructor vs.push_back(std::move(s2)); //uses String's move constructor
       
        NOTE
        Even thoughs1ands2are lvalues, the callpush_back(T&&)leaves these objects in a moved-from (empty) state. If you want these objects to retain their original state even after apush_back()call, stick withpush_back(const T&)instead.
       
       Conclusion
       The move special member functions are significantly faster than their copy counterparts. Therefore, object copying can be replaced by moving in the following cases:
       
        You're populating a container with temporaries.
        A function is returning an object by value.
        You're passing an argument by value.
       
       语义是一个的性能优势of the prominent bonuses of using C++11. If you're designing new classes, consider adding the move special member functions to them. Some of your legacy classes probably could also benefit from a C++11 facelift that includes those special member functions.

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