CODEWITHSUNDEEP
cpointers
mysql guy
mysql guy

Reputation: 245

double pointer in loop

For the following example (from mysql source code), it uses Field **field. I am in trouble to think it as a 2 dimension array of Field.

typedef struct st_table_share  
{  
  .......  
  Field **field;           
  ....  
}  

for (Field **field=table->field ; *field ; field++)  
{  
    ...

Can I think it in this way, a table contains many rows and a row contains multiple columns/fields. So *field means a row, and **field mean a table?

If that's true, for the following code

for (Field **field=table->field ; *field ; field++)

it will exit when *field is null, so how could *field be null if *field is a row. Or can I say if a row has 5 columns, and field is the first column, then field+4 is the last column field, and field+5 is the end of the column which means null, so that the for loop will exit?

Upvotes: 3

Views: 1655

Answers (4)

Igor
Igor

Reputation: 27278

Or can I say if a row has 5 columns, and field is the first column, then field+4 is the last column field

Yes, but, you can say that

and field+5 is the end of the column which means null

only if you initialized the array to be null-terminated.

And if you know how many columns your table has, you can iterate until you reach the last column instead of ; *field ;.

Upvotes: 0

David Grayson
David Grayson

Reputation: 87531

Are you sure that Field ** is a two-dimensional array?

A variable of type Field ** does not necessarily represent a two-dimensional array of Fields. It could be a pointer to a one-dimensional array of pointers to Fields. Storing an array of pointers could be more convenient than storing an array of structs for several reasons: you can allocate the memory for the structs independently, you don't have to use memcpy to add a struct to the list, and certain list operations will be less expensive. In fact, this is done in libusb, where a variable of type libusb_device ** represents a (one-dimensional) list of usb devices.

Upvotes: 1

Fred Foo
Fred Foo

Reputation: 363848

So *field means a row, and **field mean a table?

In declaration syntax, yes.

how could *field be null if *field is a row

If the authors of the code take care to preserve the invariant that field is terminated by a null pointer, as is done with argv. I.e., a table of n rows is an array of n+1 pointers, the last of which must always be null. This is a common C convention that obviates the need for an explicit count.

Upvotes: 4

shinkou
shinkou

Reputation: 5154

The loop will continue until "*field" is null. i.e. the array should have a null-terminator (a non-existing row) at the end.

Upvotes: 0

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