Re: [Log4cplus-devel] compatibility question concerning log4cplus::tstring

[Václav Z. <vha...@gm...>]

On 2 August 2012 11:00, Václav Zeman wrote:
> On 1 August 2012 17:13, John Lumby wrote:
>> Hello,   I'd like to ask about compatibility of  log4cplus::tstring between 1.0.x and 1.1.x
> As I have written in reply in another thread, my intention has always
> been to keep log4cplus source compatible between versions.
>
>>
>> An application (bind10) has some code like so:
>>
>> // Convert logging level to string.  If the level is a valid debug level,
>> // return the string DEBUG, else return the empty string.
>> log4cplus::tstring
>> LoggerLevelImpl::logLevelToString(log4cplus::LogLevel level) {
>>     Level bindlevel = convertToBindLevel(level);
>>     Severity& severity = bindlevel.severity;
>>     int& dbglevel = bindlevel.dbglevel;
>>
>>     if ((severity == DEBUG) &&
>>         ((dbglevel >= MIN_DEBUG_LEVEL) && (dbglevel <= MAX_DEBUG_LEVEL))) {
>>         return (tstring("DEBUG"));
>>     }
>>
>>     // Unknown, so return empty string for log4cplus to try other conversion
>>     // functions.
>>     return (tstring());
>> }
>>
>> // Initialization.  Register the conversion functions with the LogLevelManager.
>> void
>> LoggerLevelImpl::init() {
>>
>>     // Get the singleton log-level manager.
>>     LogLevelManager& manager = getLogLevelManager();
>>
>>     // Register the conversion functions
>>     manager.pushFromStringMethod(LoggerLevelImpl::logLevelFromString);
>>     manager.pushToStringMethod(LoggerLevelImpl::logLevelToString);
>> }
>>
>> which compiles correctly against 1.0.x
>>
>> But compiling against 1.1.x results in
>>
>> logger_level_impl.cc:189:1: error: prototype for
>>          ‘const tstring& isc::log::LoggerLevelImpl::logLevelToString(log4cplus::LogLevel)’
>>       does not match any in class ‘isc::log::LoggerLevelImpl’
>> ../../../src/lib/log/logger_level_impl.h:115:31: error:
>>         candidate is: static log4cplus::tstring isc::log::LoggerLevelImpl::logLevelToString(log4cplus::LogLevel)
>>             In static member function ‘static void isc::log::LoggerLevelImpl::init()’:
>>
>> Changing the application code to read as follows fixes the problem ...
>>
>> // Convert logging level to string.  If the level is a valid debug level,
>> // return the string DEBUG, else return the empty string.
>> log4cplus::tstring const &
>> LoggerLevelImpl::logLevelToString(log4cplus::LogLevel level) {
>>
>>     static const tstring debug_string("DEBUG");
>>     static const tstring empty_string;
>>
>>     Level bindlevel = convertToBindLevel(level);
>>     Severity& severity = bindlevel.severity;
>>     int& dbglevel = bindlevel.dbglevel;
>>
>>     if ((severity == DEBUG) &&
>>         ((dbglevel >= MIN_DEBUG_LEVEL) && (dbglevel <= MAX_DEBUG_LEVEL))) {
>>         return (debug_string);
>>     }
>>
>>     // Unknown, so return empty string for log4cplus to try other conversion
>>     // functions.
>>     return (empty_string);
>> }
>>
>> but creates two new problems:
>>   .    code is less general  -   every possible string value which might be returned must be explicitly declared
>>           (fortunately there are only two  - "DEBUG" and "")
> I do not regard this as a problem. The set of log levels is intended
> to be small. The change of return type to reference to tstring is
> there for speed.
>
>>   .  code no longer compiles against 1.0.x  -
> This is a problem.  What you have discovered breaks the source
> compatibility intention and I am going to fix it by allowing both old
> and new callbacks.
>
>> logger_level_impl.cc: In static member function 'static void
>> isc::log::LoggerLevelImpl::init()':
>> logger_level_impl.cc:217: error: invalid conversion from 'const
>> log4cplus::tstring& (*)(log4cplus::LogLevel)' to 'log4cplus::tstring
>> (*)(log4cplus::LogLevel)'
>> logger_level_impl.cc:217: error:   initializing argument 1 of 'void
>> log4cplus::LogLevelManager::pushToStringMethod(log4cplus::tstring
>> (*)(log4cplus::LogLevel))'
>>
>> Has anyone else run into this?
>> And is it possible to write the application code for registering and running the logLevelToString() method in such a way that it
>> compiles correctly against both log4cplus releases?
I am attaching a patch that should allow using callbacks with both return types.

If there is any other source incompatibility, please let me know.

-- 
VZ