Re: ExceptionHandler class & list of error messages ?
mast2as@yahoo.com wrote:
Hi I tried to "copy" your code (the names are a bit different) and i
get an error when i compile which i don't understand well... could you
help me a bit more please ? thank you...
/////// COMPILE ERROR
test.cpp: In member function `virtual const char* TException::what()
const':
test.cpp:28: error: passing `const std::map<const int, const char*,
std::less<const int>, std::allocator<std::pair<const int, const char*>
' as `this' argument of `_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = const int, _Tp = const char*, _Compare = std::less<const int>, _Alloc = std::allocator<std::pair<const int, const char*> >]' discards qualifiers
/////// CODE
#include <iostream>
#include <deque>
FYI, this header isn't used (but maybe that's because you deleted the
code that referenced it for simplicity).
#include <map>
#include <exception>
#define PRINTERROR 0
#define ABORTERROR 1
Prefer const ints or (better) an enumeration to these defines since
macros are evil (cf.
http://www.parashift.com/c++-faq-lite/inline-functions.html#faq-9.5)
and should be avoided in favor of C++ constructs when possible. These
two lines won't likely cause a problem, but it's better to be in the
habit of doing things correctly.
class TException : public std::exception
{
public:
enum errorCode {
TEST1 = 1,
TEST2,
TEST3
};
typedef std::map<const int, const char*> MsgMap;
The problem is the const on the int. The map's key type is always made
const, and so you can't add that qualifier here lest you end up with a
"const const int". However, you should change the key type to errorCode
for an added bit of type safety.
public:
TException( const int code, const int severity = PRINTERROR ) :
Change this int to errorCode, too, for type safety. It won't guarantee
that the user doesn't cast an invalid integer value into your
enumeration type, but it will be helpful for non-devious users. For
instance, this line wouldn't compile if you made this change:
throw TException( 42 );
but this line would:
throw TException( 1 );
The latter is less explicit and more prone to accidents than the
equivalent "TException( TException::TEST1 )". Prefer to use your
enumeration to help the programmer use your class correctly.
m_code( code ),
m_severity( severity )
{}
~TException() throw() {}
virtual const char* what() const throw()
{
return myMsgMap[ m_code ];//"some text";
}
There's another problem here. I didn't actually try compiling the code
before, and I overlooked the fact that the [] operator for map isn't
const, and since the map is (rightly) declared const in your class, you
can't call non-const methods on it (cf.
http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.10
and
http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.12).
The correct version of this function is:
virtual const char* what() const throw()
{
const MsgMap::const_iterator iter = myMsgMap.find( m_code );
if( iter != myMsgMap.end() )
{
return iter->second;
}
return "No error message found!";
}
public:
private:
const int m_severity;
const int m_code;
Change this to errorCode as well.
const static MsgMap myMsgMap;
};
Remember, the following lines need to be in a .cpp file somewhere, not
a header.
namespace
{
class TInitialise
{
public:
TException::MsgMap m_;
public:
operator TException::MsgMap() const { return m_; }
TInitialise() {};
TInitialise &Add() { std::cout << "in add\n"; return *this; }
};
}
const TException::MsgMap TException::myMsgMap = TInitialise()
.Add()
.Add();
int main()
{
return 0;
}
Cheers! --M