Effective C++: Item 24 (轉)

Effective C++: Item 24 (轉)[@more@]

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Item 24: Choose carefully between function overloading and parameter defaulting.

The confusion over function overloading and parameter defaulting stems from the fact that they both allow a single function name to be called in more than one way:

void f(); // f is overloaded void f(int x); f(); // calls f() f(10); // calls f(int) void g(int x = 0); // g has a default // parameter value g(); // calls g(0) g(10); // calls g(10)

So which should be used when?

The answer depends on two other questions. First, is there a value you can use for a default? Second, how many algorithms do you want to use? In general, if you can choose a reasonable default value and you want to employ only a single algorithm, you'll use default parameters (see also Item 38). Otherwise you'll use function overloading.

Here's a function to compute the maximum of up to five ints. This function uses -- take a deep breath and steel yourself -- std::numeric_limits::min() as a default parameter value. I'll have more to say about that in a moment, but first, here's the code:

int max(int a, int b = std::numeric_limits::min(), int c = std::numeric_limits::min(), int d = std::numeric_limits::min(), int e = std::numeric_limits::min()) { int temp = a > b ? a : b; temp = temp > c ? temp : c; temp = temp > d ? temp : d; return temp > e ? temp : e; }

Now, calm yourself. std::numeric_limits::min() is just the fancy new-fangled way the standard C++ library says what C says via the INT_MIN macro in : it's the minimum possible value for an int in whatever compiler happens to be processing your C++ code. True, it's a deviation from the terseness for which C is renowned, but there's a method behind all those colons and other syntactic strychnine.

Suppose you'd like to write a function template taking any built-in numeric type as its parameter, and you'd like the functions generated from the template to print the minimum value representable by their instantiation type. Your template would look something like this:

template void printMinimumValue() { cout << the minimum value representable by T; }

This is a difficult function to write if all you have to work with is and . You don't know what T is, so you don't know whether to print out INT_MIN or L_MIN or what.

To sstep these difficulties, the standard C++ library (see Item 49) defines in the header a class template, numeric_limits, which itself defines several static member functions. Each function returns information about the type instantiating the template. That is, the functions in numeric_limits return information about type int, the functions in numeric_limits return information about type double, etc. Among the functions in numeric_limits is min. min returns the minimum representable value for the instantiating type, so numeric_limits::min() returns the minimum representable integer value.

Given numeric_limits (which, like nearly everything in the standard library, is in namespace std -- see Item 28; numeric_limits itself is in the header ), writing printMinimumValue is as easy as can be:

template void printMinimumValue() { cout << std::numeric_limits::min(); }

This numeric_limits-based approach to specifying type-dependent constants may look expensive, but in fact it's not. That's because the long-windedness of the source code fails to be reflected in the resultant code. In fact, calls to functions in numeric_limits generate no instructions at all. To see how that can be, consider the following, which is an obvious way to implement numeric_limits::min:

#include namespace std { inline int numeric_limits::min() throw () { return INT_MIN; } }

Because this function is declared inline, calls to it should be replaced by its body (see Item 33). That's just INT_MIN, which is itself a simple #define for some implementation-defined constant. So even though the max function at the beginning of this Item looks like it's making a function call for each default parameter value, it's just using a clever way of referring to a type-dependent constant, in this case the value of INT_MIN. Such efficient cleverness abounds in C++'s standard library. You really should read Item 49.

Getting back to the max function, the crucial observation is that max uses the same (rather inefficient) algorithm to compute its result, regardless of the number of arguments provided by the caller. Nowhere in the function do you attempt to figure out which parameters are "real" and which are defaults. Instead, you have chosen a default value that cannot possibly affect the validity of the computation for the algorithm you're using. That's what makes the use of default parameter values a viable solution.

For many functions, there is no reasonable default value. For example, suppose you want to write a function to compute the average of up to five ints. You can't use default parameter values here, because the result of the function is dependent on the number of parameters passed in: if 3 values are passed in, you'll divide their sum by 3; if 5 values are passed in, you'll divide their sum by 5. Furthermore, there is no "magic number" you can use as a default to indicate that a parameter wasn't actually provided by the client, because all possible ints are valid values for the parameters. In this case, you have no choice: you must use overloaded functions:

int avg(int a); int avg(int a, int b); int avg(int a, int b, int c); int avg(int a, int b, int c, int d); int avg(int a, int b, int c, int d, int e);

The other case in which you need to use overloaded functions occurs when you want to accomplish a particular task, but the algorithm that you use depends on the inputs that are given. This is commonly the case with constructors: a default constructor will construct an object from scratch, whereas a copy constructor will construct one from an existing object:

// A class for representing natural numbers class Natural { public: Natural(int initValue); Natural(const Natural& rhs); private: unsigned int value; void init(int initValue); void error(const string& msg); }; inline void Natural::init(int initValue) { value = initValue; } Natural::Natural(int initValue) { if (initValue > 0) init(initValue); else error("Illegal initial value"); } inline Natural::Natural(const Natural& x) { init(x.value); }

The constructor taking an int has to perfoerror checking, but the copy constructor doesn't, so two different functions are needed. That means overloading. However, note that both functions must assign an initial value for the new object. This could lead to code duplication in the two constructors, so you maneuver around that problem by writing a private member function init that contains the code common to the two constructors. This tactic -- using overloaded functions that call a common underlying function for some of their work -- is worth remembering, because it's frequently useful (see e.g., Item 12).