Stephen A. Fuqua (saf)

a Bahá'í, software engineer, and nature lover in Austin, Texas, USA

Rediscovering C++ / Performing SQL Bulk Copy Operations

When last I worked with C++, it was while working on my master’s thesis ten years ago, using a basic text editor in a Red Hat Linux 5.0 installation. A new task in front of me: replace a Reporting Services report, which was exporting to CSV, with a new solution that will allow me to create multiple files, with max 150,000 records each. The first challenge is speed: with that many records, only bulk copy will be reasonable. The second is splitting the file. I thought about calling BCP from a C# process, because unfortunately managed code only offers bulk loading into a SQL Server database, not from database to file. But C++ is another story, thanks to the Bulk Copy Driver Extensions made available by Microsoft. So, time for a C# developer to brush up on C++, and learn it the Visual Studio way!

To get started, I found the Microsoft SQL Server Community Projects & Samples site, and the Bulk Copy functions documentation linked above. Download the project samples to get a quick-start on coding for bulk copy operations; I found the projects How to bulk copy a SELECT result set, BulkCopyFormatAndData, and How to process ODBC errors particularly useful for my goal. Creating a new Visual C++ console application, I was able to quickly stitch together a working prototype that would perform a hard-coded query, using a hard-coded ODBC connection name, and bulk-loading to a hard-coded file. It displayed any error messages at the console. In customizing the error logging, I re-taught myself about sprintf (actually, sprintf_s) and stringstream. The ODBC driver does not always provide a helpful message, so I threw together a few methods for testing whether or not I could create a file (for writing the BCP output and BCP error files) or whether or not a file exists already (for the BCP format file), using the CreateFile function for both.

Next I decided to re-learn how try/catch works in the C++ world, and chose to do so without the Microsoft _finally extension (resource 1, resource 2). I was most surprised when I found that a compilation failure was due to my use of the _new keyword, as in throw new MyException(). It also took me a few tries to get the syntax correct for overriding the what() method for my custom exception, which is inheriting from the STL’s exception class. It came down to getting the correct modifiers on the declaration in the header file, combined with a problem I ran into several times: using the expected namespace in the code file. That is, I couldn’t simply put virtual const char* what() into my code file — I needed to put virtual const char* MyException::what(). Perhaps I needed a using namespace statement to avoid this.

Garbage collection is basically taken for granted in C#, but I know it is all up to me in C++. With the help of a Stack Overflow Q&A on Memory Management in C++, I am starting to explore this arena. Perhaps creating a custom Exception class wasn’t such a good idea; I’m forgetting the cardinal rule that you don’t use it for program flow, although it sure does simplify my code (a sub-function throws an exception, and I don’t have to code for various return values after each invocation of that sub-function). Not only that, but throwing exceptions makes it more likely that you’ll end up with memory leaks. In fact, even as I’m typing this I realize that I have foolishly failed to properly de-allocate a resource (see below; would have avoided this in C# through the using (...) construct). This Q&A led me to do some searching on stack-allocated constructors, turning up When to use “new” and when not to, in C++?. Now I have some understanding of the lack of_ new_ in the exception-throwing.

	throw MyException(filePath);
CloseHandle(h); // not de-allocated if the exception is thrown!

Alternately, I could wrap this all in a try/catch, add CloseHandle(h) into the catch and re-throw the exception. Or, I could simply put the CloseHandle(h) inside the if clause, before the throw statement.

Now I’ve moved on to creating a DLL and linking that to my main executable, with the hard-coded values changed into class variables that have proper getters and setters (didn’t want to use the proprietary Properties offered by Microsoft). Running against a deadline, I may try to use this DLL from C# so that I can perform the file manipulation (splitting by 150,000) in familiar code. But how hard can it be to split a file in C++? Surely not too difficult.

Posted with : Tech, General Programming