Item 2: Prefer consts, enums, and inlines to defines

Item 2: Prefer consts, enums, and inlines to #defines

This Item might better be called "prefer the compiler to the preprocessor," because #define may be treated as if it's not part of the language per se. That's one of its problems. When you do something like this,

 #define ASPECT_RATIO 1.653 

the symbolic name ASPECT_RATIO may never be seen by compilers; it may be removed by the preprocessor before the source code ever gets to a compiler. As a result, the name ASPECT_RATIO may not get entered into the symbol table. This can be confusing if you get an error during compilation involving the use of the constant, because the error message may refer to 1.653, not ASPECT_RATIO. If ASPECT_RATIO were defined in a header file you didn't write, you'd have no idea where that 1.653 came from, and you'd waste time tracking it down. This problem can also crop up in a symbolic debugger, because, again, the name you're programming with may not be in the symbol table.

The solution is to replace the macro with a constant:

 const double AspectRatio = 1.653;   // uppercase names are usually for                                     // macros, hence the name change 

As a language constant, AspectRatio is definitely seen by compilers and is certainly entered into their symbol tables. In addition, in the case of a floating point constant (such as in this example), use of the constant may yield smaller code than using a #define. That's because the preprocessor's blind substitution of the macro name ASPECT_RATIO with 1.653 could result in multiple copies of 1.653 in your object code, while the use of the constant AspectRatio should never result in more than one copy.

When replacing #defines with constants, two special cases are worth mentioning. The first is defining constant pointers. Because constant definitions are typically put in header files (where many different source files will include them), it's important that the pointer be declared const, usually in addition to what the pointer points to. To define a constant char*-based string in a header file, for example, you have to write const twice:

 const char * const authorName = "Scott Meyers"; 

For a complete discussion of the meanings and uses of const, especially in conjunction with pointers, see Item 3. However, it's worth reminding you here that string objects are generally preferable to their char*-based progenitors, so authorName is often better defined this way:

 const std::string authorName("Scott Meyers"); 

The second special case concerns class-specific constants. To limit the scope of a constant to a class, you must make it a member, and to ensure there's at most one copy of the constant, you must make it a static member:

 class GamePlayer { private:   static const int NumTurns = 5;      // constant declaration   int scores[NumTurns];               // use of constant   ... }; 

What you see above is a declaration for NumTurns, not a definition. Usually, C++ requires that you provide a definition for anything you use, but class-specific constants that are static and of integral type (e.g., integers, chars, bools) are an exception. As long as you don't take their address, you can declare them and use them without providing a definition. If you do take the address of a class constant, or if your compiler incorrectly insists on a definition even if you don't take the address, you provide a separate definition like this:

 const int GamePlayer::NumTurns;     // definition of NumTurns; see                                     // below for why no value is given 

You put this in an implementation file, not a header file. Because the initial value of class constants is provided where the constant is declared (e.g., NumTurns is initialized to 5 when it is declared), no initial value is permitted at the point of definition.

Note, by the way, that there's no way to create a class-specific constant using a #define, because #defines don't respect scope. Once a macro is defined, it's in force for the rest of the compilation (unless it's #undefed somewhere along the line). Which means that not only can't #defines be used for class-specific constants, they also can't be used to provide any kind of encapsulation, i.e., there is no such thing as a "private" #define. Of course, const data members can be encapsulated; NumTurns is.

Older compilers may not accept the syntax above, because it used to be illegal to provide an initial value for a static class member at its point of declaration. Furthermore, in-class initialization is allowed only for integral types and only for constants. In cases where the above syntax can't be used, you put the initial value at the point of definition:

 class CostEstimate { private:   static const double FudgeFactor;       // declaration of static class   ...                                    // constant; goes in header file }; const double                             // definition of static class   CostEstimate::FudgeFactor = 1.35;      // constant; goes in impl. file 

This is all you need almost all the time. The only exception is when you need the value of a class constant during compilation of the class, such as in the declaration of the array GamePlayer::scores above (where compilers insist on knowing the size of the array during compilation). Then the accepted way to compensate for compilers that (incorrectly) forbid the in-class specification of initial values for static integral class constants is to use what is affectionately (and non-pejoratively) known as "the enum hack." This technique takes advantage of the fact that the values of an enumerated type can be used where ints are expected, so GamePlayer could just as well be defined like this:

 class GamePlayer { private:   enum { NumTurns = 5 };        // "the enum hack"   makes                                 // NumTurns a symbolic name for 5   int scores[NumTurns];         // fine   ... }; 

The enum hack is worth knowing about for several reasons. First, the enum hack behaves in some ways more like a #define than a const does, and sometimes that's what you want. For example, it's legal to take the address of a const, but it's not legal to take the address of an enum, and it's typically not legal to take the address of a #define, either. If you don't want to let people get a pointer or reference to one of your integral constants, an enum is a good way to enforce that constraint. (For more on enforcing design constraints through coding decisions, consult Item 18.) Also, though good compilers won't set aside storage for const objects of integral types (unless you create a pointer or reference to the object), sloppy compilers may, and you may not be willing to set aside memory for such objects. Like #defines, enums never result in that kind of unnecessary memory allocation.

A second reason to know about the enum hack is purely pragmatic. Lots of code employs it, so you need to recognize it when you see it. In fact, the enum hack is a fundamental technique of template metaprogramming (see Item 48).

Getting back to the preprocessor, another common (mis)use of the #define directive is using it to implement macros that look like functions but that don't incur the overhead of a function call. Here's a macro that calls some function f with the greater of the macro's arguments:

 // call f with the maximum of a and b #define CALL_WITH_MAX(a, b) f((a) > (b) ? (a) : (b)) 

Macros like this have so many drawbacks, just thinking about them is painful.

Whenever you write this kind of macro, you have to remember to parenthesize all the arguments in the macro body. Otherwise you can run into trouble when somebody calls the macro with an expression. But even if you get that right, look at the weird things that can happen:

 int a = 5, b = 0; CALL_WITH_MAX(++a, b);          // a is incremented twice CALL_WITH_MAX(++a, b+10);       // a is incremented once 

Here, the number of times that a is incremented before calling f depends on what it is being compared with!

Fortunately, you don't need to put up with this nonsense. You can get all the efficiency of a macro plus all the predictable behavior and type safety of a regular function by using a template for an inline function (see Item 30):

 template<typename T>                               // because we don't inline void callWithMax(const T& a, const T& b)    // know what T is, we {                                                  // pass by reference-to-   f(a > b ? a : b);                                // const   see Item 20 } 

This template generates a whole family of functions, each of which takes two objects of the same type and calls f with the greater of the two objects. There's no need to parenthesize parameters inside the function body, no need to worry about evaluating parameters multiple times, etc. Furthermore, because callWithMax is a real function, it obeys scope and access rules. For example, it makes perfect sense to talk about an inline function that is private to a class. In general, there's just no way to do that with a macro.

Given the availability of consts, enums, and inlines, your need for the preprocessor (especially #define) is reduced, but it's not eliminated. #include remains essential, and #ifdef/#ifndef continue to play important roles in controlling compilation. It's not yet time to retire the preprocessor, but you should definitely give it long and frequent vacations.

Things to Remember

  • For simple constants, prefer const objects or enums to #defines.

  • For function-like macros, prefer inline functions to #defines.

Effective C++ Third Edition 55 Specific Ways to Improve Your Programs and Designs
Effective C++ Third Edition 55 Specific Ways to Improve Your Programs and Designs
ISBN: 321334876
Year: 2006
Pages: 102 © 2008-2017.
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