Question

I am having troubles passing/receiving NSString pointers through function calls. I’m hoping someone can help me see what I’m doing incorrectly.

So this is from my first class…

void addTo(int pk, NSString* nam, NSString *descrip)
{    
    //open the database
    sqlite3 *db;
    db = [Item openDB:databasePath];

    printf("'%i', '%s', '%s'", pk, nam, descrip);
    //create new item with key, name, description, and database
    Item *Obj = [[Item alloc]initWithPrimaryKey:pk:nam:descrip:db];
                          .
                          .
                          .
}

And then this is the function in Item.m called as above…

- (id) initWithPrimaryKey:(NSInteger) pk :(NSString*) nam: (NSString*) descrip: (sqlite3*) db{

    printf("'%i', '%s', '%s'", pk, nam, descrip);
                         .
                         .
                         .
    return self;
}

Let’s say I call addTo with inputs 1234, “Tree”, “plant with leaves”

The print in the first code block outputs what I sent to addTo but the print in initWithPrimaryKey prints the following…

'1234', 'P?a', 'P?a' 

Why is this? Or more.. why is it not printing what I expect?

Answer 1

%s is used for char* strings, but %@ should be used with NSStrings. printf may or may not support %@, I don’t know. If not, you need to use NSString’s cStringUsingEncoding or UTF8String to convert to a char* that you can use.

initWithPrimaryKey:pk:nam:descrip:db is invalid (or at least very loopy) syntax, BTW, as is initWithPrimaryKey:(NSInteger) pk :(NSString*) nam: (NSString*) descrip: (sqlite3*) db.

It’s important to understand the difference between an NSString and a char* string. The NSString is a full-fledged object, with about 50 methods that it supports to do all sorts of neat/strange/(and occasionally)obscene things with string values. The two are not in any way interchangeable. And unlike with some C++ libraries, you can’t substitute a char* string for an NSString on a call and have automatic conversion occur.

So using "letters" for a string will not produce something usable as an NSString — you must use @"letters".