Format: type identifier & function name (parameter list and type description) {//function body } benefits: No copy of the returned value is generated in memory; (Note: It is for this reason that it is undesirable to return a reference to a local variable.) Because with the end of the local variable lifetime, the corresponding reference will also be invalidated, resulting in runtime error! Note: (1) cannot return a reference to a local variable. This article can refer to effective c++[1] Item 31. The main reason is that the local variable is destroyed after the function returns, so the returned reference becomes a "no" reference, and the program goes into an unknown state. (2) You cannot return a reference to the memory that is allocated inside the function. This article can refer to effective c++[1] Item 31. Although there is no passive destruction of local variables, there are other embarrassing situations in which the case (a reference to new allocation of memory within the function) is returned. For example, a reference returned by a function is only present as a temporary variable, not given an actual variable, and the space pointed to by the reference (allocated by new) cannot be freed, resulting in memory leak. (3) A reference to a class member can be returned, but preferably a const. This principle can be referenced by effective C++[1] Item 30. The main reason is that when an object's property is associated with a business rule, its assignment is often related to the state of some other attribute or object, so it is necessary to encapsulate the assignment in a business rule. If other objects can get a very good reference (or pointer) to the property, the simple assignment of that property will break the integrity of the business rules. (4) The stream operator overload return value is declared as the function of "reference": Stream operator << and >> the two operators often want to be used continuously, for example: cout << "Hello" << Endl; Therefore, the return value of these two operators should be a stream reference that still supports both operators. Alternative options include returning a stream object and returning a pointer to a stream object. But for a stream object to be returned, the program must recreate (copy) a new stream object, that is to say, two consecutive << operators are actually for different objects! It's not acceptable to anyone. The << operator cannot be used consecutively for returning a stream pointer. Therefore, returning a Stream object reference is the only option. This unique choice is key, it shows the importance of the reference and the irreplaceable, perhaps this is the introduction of the concept of reference in the C + + language is the reason for it. Assignment operator =. This operation, like the Fuchang operator, can be used consecutively, for example: x = j = 10, or (x=10) = 100;The return value of the assignment operator must be an lvalue so that it can continue to be assigned. The reference is therefore the only return value selection for this operator. Example 3#i nclude <iostream.h>int &put (int n), int vals[10];int error=-1;void main () {put (0) = 10;//put (0) function value as Lvalue, Equivalent to Vals[0]=10;put (9) = 20; Use the put (9) function value as the left value, equivalent to vals[9]=20;cout<<vals[0];cout<<vals[9];} int &put (int n) {if (n>=0 && n<=9) return vals[n];else {cout<< "subscript error"; return error;} } (5) In some other operators, it is not possible to return a reference: +-*/arithmetic character. They cannot return references, effective c++[1] Item23 detailed discussion of this issue. The main reason is that these four operators do not have side effect, so they must construct an object as the return value, and the optional scheme includes returning an object, returning a reference to a local variable, returning a reference to a new allocated object, and returning a static object reference. According to the preceding reference as the three rules for the return value, the 2nd and 32 scenarios are deprecated. A reference to a static object also causes an error because ((a+b) = = (C+d) is always true. So the only option left is to return an object.
The reference is not recommended in other cases where the stream operator << and >>, subscript operator [], the return value of the assignment operator =, the parameter of the copy constructor, the assignment operator = parameter, and so on.
When does C + + use references
