什么是预处理器宏有用？

Question

After reading another question about the use of macros, I wondered: What are they good for?

One thing I don't see replaced by any other language construct very soon is in diminishing the number of related words you need to type in the following:

void log_type( const bool value ) { std::cout << "bool: " << value; }
void log_type( const int value ) { std::cout << "int: " << value; }
...
void log_type( const char  value ) { std::cout << "char: "  << value; }
void log_type( const double  value ) { std::cout << "int: "  << value; }
void log_type( const float  value ) { std::cout << "float: "  << value; }

as opposed to

#define LOGFN( T ) void log_type( const T value ) { std::cout << #T ## ": " << value; }
LOGFN( int )
LOGFN( bool )
...
LOGFN( char )
LOGFN( double )
LOGFN( float )

Any other 'irreplaceables'?

EDIT: trying to summarize the reasons-why encountered in the answers; since that's what I was interested in. Mainly because I have a feeling that most of them are due to us still programming in raw text files in, still, poorly supporting environments.

• flexibility of code-to-be compiled (eg #ifdef DEBUG , platform issues) (SadSido, Catalin, Goz)
• debug purposes (eg __LINE__, __TIME__ ); I also put 'stringifying' under this reason (SadSido, Jla3ep, Jason S)
• replacing eg PHP's require vs. include feature ( #pragma once ) (SadSido, Catalin)
• readability enhancement by replacing complicated code (eg MESSAGEMAP , BOOST_FOREACH ) (SadSido, fnieto)
• DRY principle (Jason S)
• an inline replacement (Matthias Wandel, Robert S. Barnes)
• stringifying (Jason S)

Answer 1

• compile different code under different conditions ( #ifdef __DEBUG__ );
• guards to include each header once for each translation unit ( #pragma once );
• __FILE__ and __LINE__ - replaced by the current file name and current line;
• structuring the code to make it more readable (ex: BEGIN_MESSAGE_MAP() );

See interesting macro discussion at gotw here:

http://www.gotw.ca/gotw/032.htm

http://www.gotw.ca/gotw/077.htm

Answer 2

Most useful - header file guarding:

#ifndef MY_HEADER_GUARD
#define MY_HEADER_GUARD

// Header file content.

#endif 

Later add [ Windows only ]

Exporting classes to DLL:

#ifdef EXPORT_MY_LIB
#define    MY_API __declspec( dllexport)
#else
#define    MY_API __declspec( dllimport)
#endif

Sample class:

class MY_API MyClass { ... };

Answer 3

platform specific sections.

ie

#ifdef WINDOWS
#include "WindowsImplementation.h"
#elif defined( LINUX )
#include "LinuxImplementation.h"
#else
#error Platform undefined.
#endif

Answer 4

I've posted this before, but of course cannot now find it. If you want to access the __FILE__ and __LINE__ macros, then another macro is by far the most convenient way to go - for example:

#define ATHROW(msg)                                         \
{                                                           \
    std::ostringstream os;                                  \
    os << msg;                                              \
    throw ALib::Exception( os.str(), __LINE__, __FILE__ );  \
}

Answer 5

For doing cool magic tricks like in BOOST_FOREACH , injecting variables into an ambit.

BOOST_FOREACH( char c, "Hello, world!" )
{
   ... use char variable c here ...
}   // c's scope ends here
// if there's an outer c defined, its scope resumes here

Answer 6

For don't-repeat-yourself (DRY) reasons. Things that involve repeated constructs at compile-time which cannot be abstracted away in other methods (templates or what have you). If you are finding you're repeating the same code constructs 20 times, that's a potential source of human error -- which hopefully can be abstracted away using templates but sometimes not. It's always a balance between the advantages of seeing raw code that can be type-checked and reviewed clearly, vs. the advantages of using macros for arbitrary substitution patterns (that generally can't be checked by automatic programming tools).

Stringifying and concatenation (the # and ## preprocessor patterns) can't be performed by templates.

Of course, at some point you may be better off using a tool (whether custom or off-the-shelf) for automatic code generation.

Answer 7

Modern languages take the philosophy that needing a proeprocessor ins a sign of a missing language feature, so they define all kinds of language features that the preprocessor took care of very simply back in the old K&R style C.

Your code example above could be simplified via an inline function, for example.

Personally, the most indispensable aspect of a preprocessor is to make it clear that some things are done compile time right in the source code. The java approach of eliminating dead code paths at compile time is just not as obvious when reading the code.

Answer 8

One of their uses is basically as a poor mans ( inlined ) template function.

For example :

#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))

This allows you to generate a custom MIN function for any types supporting these operators which is effectively inline at the point of useage. Of course there is no type checking and it's easy to end up with weird syntax errors or incorrect behavior if you don't get the parens just right.