宏展开试例

#include <iostream>
#include <cstdarg>
#include <cstdio>

using namespace std;


int ImplLogPrint(int iLogLevel, const char *format, va_list argList);

#define ERROR 0
#define WARNING 1
#define INFO    2
#define DEBUG   3

#define _IMPL_LOGPRINT(LogLevel) \
        void LogPrint##LogLevel(const char *format, ...) \
        { \
            va_list argList; \
            va_start(argList, format); \
            ImplLogPrint(LogLevel, format, argList); \
            va_end(argList);\
        }

#define IMPL_LOGPRINT(x) _IMPL_LOGPRINT(x) //跳板宏，用于展开ERROR 等宏

IMPL_LOGPRINT(ERROR) //展开之后为LogPrint0
IMPL_LOGPRINT(WARNING)
IMPL_LOGPRINT(INFO)
IMPL_LOGPRINT(DEBUG)

#define Logging(LogLevel) (LogLevel <= GetLogLevel())

#define _LogEvent(LogLevel, x) \
do { \
    if (Logging(LogLevel)) \
    { \
        LogPrint##LogLevel x ; \
    } \
} while(0)

#define LogEvent(LogLevel, x) _LogEvent(LogLevel, x) //跳板宏，用于展开ERROR 等宏

#define LogError(x) LogEvent(ERROR, x)
#define LogWarning(x) LogEvent(WARNING, x)
#define LogInfo(x) LogEvent(INFO, x)
#define LogDebug(x) LogEvent(DEBUG, x)

int g_LogLevel = 0;

int GetLogLevel()
{
    return g_LogLevel;
}

int SetLogLevel(int iLevel)
{
    g_LogLevel = iLevel;

    return 0;
}

int ImplLogPrint(int iLogLevel, const char *format, va_list argList)
{
    char buf[1024];

    int ret = vsprintf(buf, format, argList);

    switch(iLogLevel)
    {
    case ERROR:
        cout << "Error:"<< buf << endl;
        break;
    case WARNING:
        cout << "Warning:"<< buf << endl;
        break;
    case INFO:
        cout << "Info:"<< buf << endl;
        break;
    case DEBUG:
        cout << "Debug:"<< buf << endl;
        break;
    default:
        break;
    }

    return ret;
}

int main()
{
    SetLogLevel(3);
    LogError(("hello %s", "world"));
    LogWarning(("hello %s", "world"));
    LogInfo(("hello %s", "world"));
    LogDebug(("hello %s", "world"));

    SetLogLevel(1);
    LogError(("hello %s", "world"));
    LogWarning(("hello %s", "world"));
    LogInfo(("hello %s", "world"));
    LogDebug(("hello %s", "world"));

    return 0;
}

预编译展开后的代码 
# g++ -E debug_level.cpp  -o debug_level.i