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.