More Effective C++ | Item 5: Be wary of user-defined conversion functions Back to Operators 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 doubles (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 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 objects and some code that uses such objects: bool operator==( const Array& lhs, const Array& rhs); Array a(10); Array 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 (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 object by calling the Array 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 >(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 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 Array { public: ... explicit Array(int size); // note use of "explicit" ... }; Array a(10); // okay, explicit ctors can // be used as usual for // object construction Array b(10); // also okay if (a == b[i]) ... // error! no way to // implicitly convert // int to Array if (a == Array(b[i])) ... // okay, the conversion // from int to Array is // explicit (but the logic of // the code is suspect) if (a == static_cast< Array >(b[i])) ... // equally okay, equally // suspect if (a == (Array)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>(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 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 a(10); Your compilers are asked to call a constructor in the Array 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 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& lhs, const Array& rhs); Array a(10); Array 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 on the right-hand side of the "==" in order to call operator== for Array 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 object from this temporary, because that would call for two user-defined conversions, one from int to ArraySize and one from ArraySize to Array. 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