cppcms-users

[Cppcms-users] Cppdb: How to fetch a column of unknown type at compile time?

[Daniel V. <chi...@gm...>]

How to fetch a column of unknown type at compile time with cppdb?

For example in soci we can do code like this:

        row r;
        sql << "select arbitrary_column from arbitrary_table where id =
        1", into(r);
        
        const column_properties & props = r.get_properties(0);
        
        switch(props.get_data_type())
        {
        case dt_string:
            cout << r.get<std::string>(i);
            break;
        case dt_integer:
            cout << r.get<int>(i);
            break;

    	//others cases...
        
        }

What is the recommended way to accomplish this with cppdb? ( If one
exists).


Thanks.

Re: [Cppcms-users] Cppdb: How to fetch a column of unknown type at compile time?

[Artyom <art...@ya...>]

> 
> How to fetch a column of unknown type at compile time with cppdb?
> 

You can always fetch it as string, any value (with small exception of Binary 
Large OBjects)
is convertible to string and actually under the hood it is almost always string.

Why there is no functionality to get the type of the column?

Because it is not always well defined. With some databases like sqlite3
the data is "typeless" and even sometimes when you fetch a column
it may have some "original" type it does not promises that it
would be actually the type you are trying to use or it may be
even undefined as it is some combined value.

Bottom line:

- CppDB is dynamically typed and try to perform the conversions
  in the best way it can.

- If you need to check whether specific value can be treated as
  for example integer, you can do following:

      try {
        int x = r.get<int>(v);
      }
      catch(cppdb::bad_value_cast const &e) {
        // ok it can't be integer
      }
  
   But of course of the original type is string and it holds the text
   "123" as string, cppdb would happily cast if for you.


Artyom


      

Re: [Cppcms-users] Cppdb: How to fetch a column of unknown type at compile time?

[Klaim <mj...@gm...>]

Hi,
reading this example

 try {
>        int x = r.get<int>(v);
>      }
>      catch(cppdb::bad_value_cast const &e) {
>        // ok it can't be integer
>      }
>

I'm thinking : could it be possible to provide a way to make it return a
specific value instead of throwing when the conversion can't be done?
I'm not sure that would be useful in this case but it have been when using
boost::property_tree for example.
Something like

 int x = r.get<int>( v ,  -1 ); // return -1 if something gone wrong

Alllowing the two syntaxes help avoiding unnecessary try/catch when you just
want an obviously wrong result instead of a potential exception you'll have
to remember to catch to stop crashing.

Re: [Cppcms-users] Cppdb: How to fetch a column of unknown type at compile time?

[Artyom <art...@ya...>]

>
>From: Klaim <mj...@gm...>
>To: cpp...@li...
>Sent: Tue, January 4, 2011 12:18:59 PM
>Subject: Re: [Cppcms-users] Cppdb: How to fetch a column of unknown type at 
>compile time?
>
>Hi,
>reading this example
>
>
> try {
>>       int x = r.get<int>(v);
>>     }
>>     catch(cppdb::bad_value_cast const &e) {
>>       // ok it can't be integer
>>     }
>
>I'm thinking : could it be possible to provide a way to make it return a 
>specific value instead of throwing when the conversion can't be done?
>I'm not sure that would be useful in this case but it have been when using 
>boost::property_tree for example.
>Something like 
>
>
> int x = r.get<int>( v ,  -1 ); // return -1 if something gone wrong
> 
>Alllowing the two syntaxes help avoiding unnecessary try/catch when you just 
>want an obviously wrong result instead of a potential exception you'll have to 
>remember to catch to stop crashing.
>

Generally you can do following:

   int x = -1;
   r.fetch(v,x);


HOWEVER this works only when the value is NULL value, in case of impossible
type conversion bad_value_cast would be thrown.

If you don't want the bad_value_cast being thrown when you don't know the type,
just fetch the value as string and manipulate over it.

Artyom