mi5.htm

一个非常适合初学者入门的有关c++的文档

We intended to compare each element of a to the corresponding element in b, but we accidentally omitted the subscripting syntax when we typed a. Certainly we expect this to elicit all manner of unpleasant commentary from our compilers, but they will complain not at all. That's because they see a call to operator== with arguments of type Array<int> (for a) and int (for b[i]), and though there is no operator== function taking those types, our compilers notice they can convert the int into an Array<int> object by calling the Array<int> constructor that takes a single int as an argument. This they proceed to do, thus generating code for a program we never meant to write, one that looks like this: for (int i = 0; i < 10; ++i)
  if (a == static_cast< Array<int> >(b[i]))   ...


Each iteration through the loop thus compares the contents of a with the contents of a temporary array of size b[i] (whose contents are presumably undefined). Not only is this unlikely to behave in a satisfactory manner, it is also tremendously inefficient, because each time through the loop we both create and destroy a temporary Array<int> object (see Item 19).
The drawbacks to implicit type conversion operators can be avoided by simply failing to declare the operators, but single-argument constructors cannot be so easily waved away. After all, you may really want to offer single-argument constructors to your clients. At the same time, you may wish to prevent compilers from calling such constructors indiscriminately. Fortunately, there is a way to have it all. In fact, there are two ways: the easy way and the way you'll have to use if your compilers don't yet support the easy way.
The easy way is to avail yourself of one of the newest C++ features, the explicit keyword. This feature was introduced specifically to address the problem of implicit type conversion, and its use is about as straightforward as can be. Constructors can be declared explicit, and if they are, compilers are prohibited from invoking them for purposes of implicit type conversion. Explicit conversions are still legal, however: template<class T>
class Array {
public:
  ...
  explicit Array(int size);                  // note use of "explicit"
  ...
};

Array<int> a(10);                            // okay, explicit ctors can
                                             // be used as usual for
                                             // object construction

Array<int> b(10);                            // also okay

if (a == b[i]) ...                           // error! no way to
                                             // implicitly convert
                                             // int to Array<int>

if (a == Array<int>(b[i])) ...               // okay, the conversion
                                             // from int to Array<int> is
                                             // explicit (but the logic of
                                             // the code is suspect)

if (a == static_cast< Array<int> >(b[i])) ...
                                             // equally okay, equally
                                             // suspect

if (a == (Array<int>)b[i]) ...               // C-style casts are also
                                             // okay, but the logic of
                                             // the code is still suspect

In the example using static_cast (see Item 2), the space separating the two ">" characters is no accident. If the statement were written like this, if (a == static_cast<Array<int>>(b[i])) ...


it would have a different meaning. That's because C++ compilers parse ">>" as a single token. Without a space between the ">" characters, the statement would generate a syntax error.
If your compilers don't yet support explicit, you'll have to fall back on home-grown methods for preventing the use of single-argument constructors as implicit type conversion functions. Such methods are obvious only after you've seen them.
I mentioned earlier that there are complicated rules governing which sequences of implicit type conversions are legitimate and which are not. One of those rules is that no sequence of conversions is allowed to contain more than one user-defined conversion (i.e., a call to a single-argument constructor or an implicit type conversion operator). By constructing your classes properly, you can take advantage of this rule so that the object constructions you want to allow are legal, but the implicit conversions you don't want to allow are illegal.
Consider the Array template again. You need a way to allow an integer specifying the size of the array to be used as a constructor argument, but you must at the same time prevent the implicit conversion of an integer into a temporary Array object. You accomplish this by first creating a new class, ArraySize. Objects of this type have only one purpose: they represent the size of an array that's about to be created. You then modify Array's single-argument constructor to take an ArraySize object instead of an int. The code looks like this: template<class T>
class Array {
public:

  class ArraySize {                            // this class is new
  public:
    ArraySize(int numElements): theSize(numElements) {}
    int size() const { return theSize; }

private:
    int theSize;
  };

Array(int lowBound, int highBound);
  Array(ArraySize size);                     // note new declaration

...

};


Here you've nested ArraySize inside Array to emphasize the fact that it's always used in conjunction with that class. You've also made ArraySize public in Array so that anybody can use it. Good.
Consider what happens when an Array object is defined via the class's single-argument constructor: Array<int> a(10);


Your compilers are asked to call a constructor in the Array<int> class that takes an int, but there is no such constructor. Compilers realize they can convert the int argument into a temporary ArraySize object, and that ArraySize object is just what the Array<int> constructor needs, so compilers perform the conversion with their usual gusto. This allows the function call (and the attendant object construction) to succeed.
The fact that you can still construct Array objects with an int argument is reassuring, but it does you little good unless the type conversions you want to avoid are prevented. They are. Consider this code again: bool operator==( const Array<int>& lhs,
                 const Array<int>& rhs);

Array<int> a(10);
Array<int> b(10);

...

for (int i = 0; i < 10; ++i)
  if (a == b[i]) ...                          // oops! "a" should be "a[i]";
                                              // this is now an error


Compilers need an object of type Array<int> on the right-hand side of the "==" in order to call operator== for Array<int> objects, but there is no single-argument constructor taking an int argument. Furthermore, compilers cannot consider converting the int into a temporary ArraySize object and then creating the necessary Array<int> object from this temporary, because that would call for two user-defined conversions, one from int to ArraySize and one from ArraySize to Array<int>. Such a conversion sequence is verboten, so compilers must issue an error for the code attempting to perform the comparison.
The use of the ArraySize class in this example might look like a special-purpose hack, but it's actually a specific instance of a more general technique. Classes like ArraySize are often called proxy classes, because each object of such a class stands for (is a proxy for) some other object. An ArraySize object is really just a stand-in for the integer used to specify the size of the Array being created. Proxy objects can give you control over aspects of your software's behavior in this case implicit type conversions that is otherwise beyond your grasp, so it's well worth your while to learn how to use them. How, you might wonder, can you acquire such learning? One way is to turn to Item 30; it's devoted to proxy classes.
Before you turn to proxy classes, however, reflect a bit on the lessons of this Item. Granting compilers license to perform implicit type conversions usually leads to more harm than good, so don't provide conversion functions unless you're sure you want them. Back to Operators
Continue to Item 6: Distinguish between prefix and postfix forms of increment and decrement operators