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• Why is the compiler not selecting my function-template overload in the following example?

  - by Steve Guidi

  Given the following function templates: #include <vector> #include <utility> struct Base { }; struct Derived : Base { }; // #1 template <typename T1, typename T2> void f(const T1& a, const T2& b) { }; // #2 template <typename T1, typename T2> void f(const std::vector<std::pair<T1, T2> >& v, Base* p) { }; Why is it that the following code always invokes overload #1 instead of overload #2? void main() { std::vector<std::pair<int, int> > v; Derived derived; f(100, 200); // clearly calls overload #1 f(v, &derived); // always calls overload #1 } Given that the second parameter of f is a derived type of Base, I was hoping that the compiler would choose overload #2 as it is a better match than the generic type in overload #1. Are there any techniques that I could use to rewrite these functions so that the user can write code as displayed in the main function (i.e., leveraging compiler-deduction of argument types)?

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• How do I truncate a .NET string?

  - by Steve Guidi

  I apologize for such a question that likely has a trivial solution, but I strangely could not find a concise API for this problem. Essentially, I would like to truncate a string such that it its length is not longer than a given value. I am writing to a database table and want to ensure that the values I write meet the constraint of the column's datatype. For instance, it would be nice if I could write the following: string NormalizeLength(string value, int maxLength) { return value.Substring(0, maxLength); } Unfortunately, this raises an exception because maxLength exceeds the string boundaries. Of course, I could write a function like the following, but I was hoping that something like this already exists. string NormalizeLength(string value, int maxLength) { return value.Length <= maxLength ? value : value.Substring(0, maxLength); } Where is the elusive API that performs this task? Is there one?

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• How do I count the number of bytes read by TextReader.ReadLine()?

  - by Steve Guidi

  I am parsing a very large file of records (one per line, each of varying length), and I'd like to keep track of the number of bytes I've read in the file so that I may recover in the event of a failure. I wrote the following: string record = myTextReader.ReadLine(); bytesRead += record.Length; ParseRecord(record); However this doesn't work since ReadLine() strips any CR/LF characters in the line. Furthermore, a line may be terminated by either CR, LF, or CRLF characters, which means I can't just add 1 to bytesRead. Is there an easy way to get the actual line length, or do I write my own ReadLine() method in terms of the granular Read() operations?

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• Which isolation level should I use for the following insert-if-not-present transaction?

  - by Steve Guidi

  I've written a linq-to-sql program that essentially performs an ETL task, and I've noticed many places where parallelization will improve its performance. However, I'm concerned about preventing uniquness constraint violations when two threads perform the following task (psuedo code). Record CreateRecord(string recordText) { using (MyDataContext database = GetDatabase()) { Record existingRecord = database.MyTable.FirstOrDefault(record.KeyPredicate()); if(existingRecord == null) { existingRecord = CreateRecord(recordText); database.MyTable.InsertOnSubmit(existingRecord); } database.SubmitChanges(); return existingRecord; } } In general, this code executes a SELECT statement to test for record existance, followed by an INSERT statement if the record doesn't exist. It is encapsulated by an implicit transaction. When two threads run this code for the same instance of recordText, I want to prevent them from simultaneously determining that the record doesn't exist, thereby both attempting to create the same record. An isolation level and explicit transaction will work well, except I'm not certain which isolation level I should use -- Serializable should work, but seems too strict. Is there a better choice?

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