Objective-C字符串

Question

I am reviewing code that makes the following call:

id<PLResultSet> results = [sqliteDatabase executeQuery:@"select * where id=?",Id];

sqliteDatabase is an instance of PlausibleDatabase (from GoogleCode, I gather). The code works, but I don't understand it. Specifically, how does this section work?

@"select * where id=?",Id

Is the query being made with ? being replaced by Id ? Or is the exeuteQuery function somehow combining the strings? How does this syntax make sense.

(Yes, I am new to Obj-C)

Thanks,

KF

Answer 1

This bit:

@"select * where id=?"

is an NSString (as opposed to a c-style string ) which is being passed into a executeQuery: : method on the sqliteDatabase object. The second (unnamed) argument to the method is Id , presumably a local variable.

Guessing from the name of the method, the sqlite wrapper probably creates a parameterized query . The question mark is the syntax used by sqlite to mark where to insert the parameters.

Answer 2

It's specific to the method executeQuery , in which ? is used as a placeholder, and the appropriate positional argument is used as filler for that placeholder (with quoting, etc. added as necessary).