cppcms-users

[Cppcms-users] statement::bind can be deceptive/dangerous

[<ele...@ex...>]

I was trying to figure out why

st.prepare(....)
....
st.bind(5, r.get<string>("date_action"));
....
st.exec()

in my code produces unpredictable behaviour untill I read in the
documentation that -

Note: the reference to the string MUST remain valid until the statement is
queried or executed!

In my code im copying some old rows into new rows and
r.get<string>("date_action") returns the letter "R" but after it got
binded to the new statement it was showing "l" which puzzled me to no-end
as to why, but if I did

string wtf = r.get<string>("date_action");
st.bind(5, wtf);

it worked fine.

Should statement::bind be using value instead of reference semantics?

Petr

Re: [Cppcms-users] statement::bind can be deceptive/dangerous

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

----- Original Message -----
> From: "ele...@ex..." <ele...@ex...>
> 
> I was trying to figure out why
> 
> st.prepare(....)
> ....
> st.bind(5, r.get<string>("date_action"));
> ....
> st.exec()
> 
> in my code produces unpredictable behaviour untill I read in the
> documentation that -
> 
> Note: the reference to the string MUST remain valid until the statement is
> queried or executed!
> 
> [ snip ]
> 
> Should statement::bind be using value instead of reference semantics?
> 


No, this is a very important performance optimization.

Consider the string of size of 50k - like article or
some other data. Copying `std::string` would require
both memory allocation and memory copy.

More than that when you work with multiple strings each
such copy semantics triggers a memory allocation that 
costs a lot, at least with C++11 string semantics 
were Copy-On-Write is forbidden.

That is why you should keep the reference valid.

BTW: You can do all this in one line

  sql << "INSERT ..." << ... << r.get<string>("date_action")) << ... << cppdb::exec;

This would be valid as the return value remains withing the statement.

Artyom Beilis.

Re: [Cppcms-users] statement::bind can be deceptive/dangerous

[<ele...@ex...>]

 >   sql << "INSERT ..." << ... << r.get<string>("date_action")) << ... <<
> cppdb::exec;
>
> This would be valid as the return value remains withing the statement.
>
> Artyom Beilis.

I understand that however what if you need to do something like this -

cppdb::statement st;
st = sql.prepare(...)

st << one << two;

if (condition is true)
   st << three << four
else
   st << five << six
end

st.exec()

In this case do I have to store all the variables on the function stack
before passing them to statement?

Petr

Re: [Cppcms-users] statement::bind can be deceptive/dangerous

[Marcel H. <ke...@co...>]

Am 07.05.2012 09:57, schrieb ele...@ex...:
>   >     sql<<  "INSERT ..."<<  ...<<  r.get<string>("date_action"))<<  ...<<
>> cppdb::exec;
>>
>> This would be valid as the return value remains withing the statement.
>>
>> Artyom Beilis.
> I understand that however what if you need to do something like this -
>
> cppdb::statement st;
> st = sql.prepare(...)
>
> st<<  one<<  two;
>
> if (condition is true)
>     st<<  three<<  four
> else
>     st<<  five<<  six
> end
>
> st.exec()
>
> In this case do I have to store all the variables on the function stack
> before passing them to statement?
if you have your elements in a vector/array/json::array(is a vector 
itself) you can do this:
http://cppcms.com/sql/cppdb/stat.html#stat_reset

> Petr
>
>
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Re: [Cppcms-users] statement::bind can be deceptive/dangerous

[<ele...@ex...>]

>> In this case do I have to store all the variables on the function stack
>> before passing them to statement?
> if you have your elements in a vector/array/json::array(is a vector
> itself) you can do this:
> http://cppcms.com/sql/cppdb/stat.html#stat_reset

This actually brings up another question about transactions.

I understand transactions work this way -

cppdb::transaction tr(sql)
sql << "INSERT INTO ......" << cppdb::exec
sql << "INSERT INTO ......" << cppdb::exec
tr.commit()

What if you can't use the syntatic sugar but have to deal with statements
directly? Ive tried

cppdb::transaction tr(sql)
cppdb::statement st = sql.prepare("...");
st.bind(...)
st.bind(...)
st.exec()

cppdb::statement st2 = sql.prepare("...")
st.bind(...)
st.exec()

tr.commit()

This throws an exception saying

sqlite3:cannot rollback transaction - SQL statements in progress

What's the proper way?