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Continue to Item 6: Distinguish between prefix and postfix forms of increment and decrement operators

Item 5: Be wary of user-defined conversion functions.

C++ allows compilers to perform implicit conversions between types. In honor of its C heritage, for example, the language allows silent conversions from `char` to `int` and from `short` to `double`. This is why you can pass a `short` to a function that expects a `double` and still have the call succeed. The more frightening conversions in C — those that may lose information — are also present in C++, including conversion of `int` to `short` and `double` to (of all things) `char`.

You can't do anything about such conversions, because they're hard-coded into the language. When you add your own types, however, you have more control, because you can choose whether to provide the functions compilers are allowed to use for implicit type conversions.

Two kinds of functions allow compilers to perform such conversions: single-argument constructors and implicit type conversion operators. A single-argument constructor is a constructor that may be called with only one argument. Such a constructor may declare a single parameter or it may declare multiple parameters, with each parameter after the first having a default value. Here are two examples:

    class Name {                                 // for names of things
    public:
      Name(const string& s);                     // converts string to
                                                 // Name
      ...

    };

    class Rational {                             // for rational numbers
    public:
      Rational(int numerator = 0,                // converts int to
               int denominator = 1);             // Rational
      ...

    };

An implicit type conversion operator is simply a member function with a strange-looking name: the word `operator` followed by a type specification. You aren't allowed to specify a type for the function's return value, because the type of the return value is basically just the name of the function. For example, to allow `Rational` objects to be implicitly converted to `double`s (which might be useful for mixed-mode arithmetic involving `Rational` objects), you might define class `Rational` like this:

    class Rational {
    public:
      ...
      operator double() const;                   // converts Rational to
    };                                           // double

This function would be automatically invoked in contexts like this:

    Rational r(1, 2);                            // r has the value 1/2

    double d = 0.5 * r;                          // converts r to a double,
                                                 // then does multiplication

Perhaps all this is review. That's fine, because what I really want to explain is why you usually don't want to provide type conversion functions of any ilk.

The fundamental problem is that such functions often end up being called when you neither want nor expect them to be. The result can be incorrect and unintuitive program behavior that is maddeningly difficult to diagnose.

Let us deal first with implicit type conversion operators, as they are the easiest case to handle. Suppose you have a class for rational numbers similar to the one above, and you'd like to print `Rational` objects as if they were a built-in type. That is, you'd like to be able to do this:

    Rational r(1, 2);

    cout << r;                                    // should print "1/2"

Further suppose you forgot to write an `operator<<` for `Rational` objects. You would probably expect that the attempt to print `r` would fail, because there is no appropriate `operator<<` to call. You would be mistaken. Your compilers, faced with a call to a function called `operator<<` that takes a `Rational`, would find that no such function existed, but they would then try to find an acceptable sequence of implicit type conversions they could apply to make the call succeed. The rules defining which sequences of conversions are acceptable are complicated, but in this case your compilers would discover they could make the call succeed by implicitly converting `r` to a `double` by calling `Rational`::`operator` `double`. The result of the code above would be to print `r` as a floating point number, not as a rational number. This is hardly a disaster, but it demonstrates the disadvantage of implicit type conversion operators: their presence can lead to the wrong function being called (i.e., one other than the one intended).

The solution is to replace the operators with equivalent functions that don't have the syntactically magic names. For example, to allow conversion of a `Rational` object to a `double`, replace `operator` `double` with a function called something like `asDouble`:

    class Rational {
    public:
      ...
      double asDouble() const;                   // converts Rational
    };                                           // to double

Such a member function must be called explicitly:

    Rational r(1, 2);

    cout << r;                                   // error! No operator<<
                                                 // for Rationals

    cout << r.asDouble();                        // fine, prints r as a
                                                 // double

In most cases, the inconvenience of having to call conversion functions explicitly is more than compensated for by the fact that unintended functions can no longer be silently invoked. In general, the more experience C++ programmers have, the more likely they are to eschew type conversion operators. The members of °the committee working on the standard C++ library (see Item E49 and Item 35), for example, are among the most experienced in the business, and perhaps that's why the `string` type they added to the library contains no implicit conversion from a `string` object to a C-style `char*`. Instead, there's an explicit member function, `c_str`, that performs that conversion. Coincidence? I think not.

Implicit conversions via single-argument constructors are more difficult to eliminate. Furthermore, the problems these functions cause are in many cases worse than those arising from implicit type conversion operators.

As an example, consider a class template for array objects. These arrays allow clients to specify upper and lower index bounds:

    template<class T>
    class Array {
    public:
      Array(int lowBound, int highBound);
      Array(int size);

      T& operator[](int index);

      ...

    };

The first constructor in the class allows clients to specify a range of array indices, for example, from 10 to 20. As a two-argument constructor, this function is ineligible for use as a type-conversion function. The second constructor, which allows clients to define `Array` objects by specifying only the number of elements in the array (in a manner similar to that used with built-in arrays), is different. It can be used as a type conversion function, and that can lead to endless anguish.

For example, consider a template specialization for comparing `Array<int>` objects and some code that uses such objects:

    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]"
        do something for when
        a[i] and b[i] are equal;
      }
      else {
        do something for when they're not;
      }

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