Re: [Design] Should accessors throw exception ?
In article <48cade74$0$196$e4fe514c@news.xs4all.nl>,
Ron AF Greve <ron@localhost> wrote:
Hi,
I think returning a special value like end() or 0 for a pointer is the best
solution.
I think exception should only be used when something exceptional occurs,
This is the key.
Is it exceptional for a name not to be found in the Yellow Pages? I
suggest no. Then in that case, you should probably have a sentinel or
return a status and the name in a out parameter.
Alternatively, you can choose to decree that it is a break of contract
to call Get() with a name that doesn't exist and supply a Find(),
and require clients to call it before calling a Get() that throw. In
that case throwing would be acceptable as it would be a break of
contract.
Most commonly however, thorwing would be used if it was exceptional to
ask for an unknown name. Say for example, an order processing
system. The client has a list of all available products that exist in
the company book, there's an interface to get stock. It would then be
sensible for size_t GetStock(std::string &productName) to throw if
product name is not in it's database since the client should only ever
request stock level for a product that is in the catalog.
Alternatively a message protocol between client and server, where the
start of each message is a message type key string that must be common
between server and client application in a private network. Message
parsing could wish to lookup how to parse the rest of the message in a
table according to the type key and throw an exception if the key is
invalid as such a message should never be in the network.
for
instance you expect a conifiguration file is there yet when you try to open
it it is missing or you try to access a database but the DBA took it down
for maintenance. This way you (or the user of your library) can catch those
thiings somewhere 'high up' in the code knowing that everything in between
is released and handled nicely and continue (maybe waiting a few minutes and
try again.
And indeed a find/get combination is more work. Also with the end() you make
it work the same as the STL.
Regards, Ron AF Greve
http://www.InformationSuperHighway.eu
"mathieu" <mathieu.malaterre@gmail.com> wrote in message
news:32642e01-cd96-49fc-9c22-5753d083ba5a@k13g2000hse.googlegroups.com...
Hi there,
I'd like to know if there is a general answer to the following
question: when making a public interface that access a container
should the function throw an exception when the element is not found ?
Otherwise I need two functions: a Find and a Get function which put
the burden on the application programmer to always call Find before
Get.
The other alternative would be to return some kind of sentinel (like
std::set<>::end() )...
Comments ?
Thanks
-Mathieu
eg.
#include <map>
#include <string.h>
struct Phone { int P; };
struct Name { const char *N;
bool operator <(Name const &n) const { return strcmp(N,n.N) < 0; }
};
struct YellowPage
{
void Insert(Name const &n,Phone const &p)
{ I.insert( std::map<Name,Phone>::value_type(n,p) ); }
bool Find(Name const & n) { return I.find(n) != I.end(); }
Phone const &Get(Name const&n) { return I.find(n)->second; }
private:
std::map<Name,Phone> I;
};