issue getting index of an array with ref arg

[c7zn]

Describe the bug

When trying to access an array, you're able to use a &int as an index by creating a new &int that equals your old &int value. This doesn't usually work as the compiler prevents &int's from being used as an index

pub fn read_int(bytes []u8, mut offset &int) int {
	defer {
		offset += 4
	}

        offset_d := offset
	return bytes[offset_d] | ( int(bytes[offset_d+ 1]) << 8 ) | ( int(bytes[offset_d+ 2]) << 16 ) | ( int(bytes[offset_d + 3]) << 24 )
}

Expected Behavior

error: non-integer index int

Current Behavior

Instead, it returns the value stored at that index.

Reproduction Steps

pub fn read_int(bytes []u8, mut offset &int) int {
	defer {
		offset += 4
	}

        offset_d := offset
	return bytes[offset_d] | ( int(bytes[offset_d+ 1]) << 8 ) | ( int(bytes[offset_d+ 2]) << 16 ) | ( int(bytes[offset_d + 3]) << 24 )
}

The above code results in zero errors, while the below code gives an error.

pub fn read_int(bytes []u8, mut offset &int) int {
	defer {
		offset += 4
	}
	return bytes[offset] | ( int(bytes[offset + 1]) << 8 ) | ( int(bytes[offset + 2]) << 16 ) | ( int(bytes[offset + 3]) << 24 ) // error: non-integer index `int` 
}

reader/file_reader.v:6:14: error: non-integer index `int` (array type `[]u8`)
    4 | 
    5 |
    6 |     return bytes[offset] | ( int(bytes[offset + 1]) << 8 ) | ( int(bytes[offset + 2]) << 16 ) | ( int(bytes[offset + 3]) << 24 )
      |                 ~~~~~~~~
    7 | }
    8 |

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.3.3 130f35c.5e48817

Environment details (OS name and version, etc.)

windows, Microsoft Windows 11 Home v22621 64-bit

[thomas-mangin]

You are passing a pointer, you need to de-reference it to use it.
Doing it "auto-magically" could break code which does want to use the pointer value.

pub fn read_int(bytes []u8, mut offset &int) int {
	defer {
		offset += 4
	}

	return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`
}

fn main() {
	mut offset := 0
	println(read_int('\1\0\0\0\2\0\0\0'.bytes(),mut &offset))
	println(offset)
}

❯ v run bug-offset.v
bug-offset.v:6:28: notice: shifting a value from a signed type `int` can change the sign
    4 |     }
    5 | 
    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`
      |                               ~~~~~~~~~~~~~~~~~~~~~~~
    7 | }
    8 |
bug-offset.v:6:63: notice: shifting a value from a signed type `int` can change the sign
    4 |     }
    5 | 
    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`
      |                                                                  ~~~~~~~~~~~~~~~~~~~~~~~
    7 | }
    8 |
bug-offset.v:6:99: notice: shifting a value from a signed type `int` can change the sign
    4 |     }
    5 | 
    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`
      |                                                                                                      ~~~~~~~~~~~~~~~~~~~~~~~
    7 | }
    8 |
1
4

So I am not sure why the affectation works ... mistery to me.
It may be related to #17788

[thomas-mangin]

Also the V (or Go) pattern for this would be:

struct Reader {
mut:
	offset int
}

pub fn (mut reader Reader) read_int(bytes []u8) int {
	defer {
		reader.offset += 4
	}
	offset := reader.offset
	return bytes[offset] | ( int(bytes[offset + 1]) << 8 ) | ( int(bytes[offset + 2]) << 16 ) | ( int(bytes[offset + 3]) << 24 ) // error: non-integer index `int`
}

fn main() {
	mut r := Reader{
		offset: 0
	}
	println(r.read_int('\1\0\0\0\2\0\0\0'.bytes()))
	println(r.read_int('\1\0\0\0\2\0\0\0'.bytes()))
}

[c7zn]

You are passing a pointer, you need to de-reference it to use it.

Doing it "auto-magically" could break code which does want to use the pointer value.

pub fn read_int(bytes []u8, mut offset &int) int {

	defer {

		offset += 4

	}



	return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`

}



fn main() {

	mut offset := 0

	println(read_int('\1\0\0\0\2\0\0\0'.bytes(),mut &offset))

	println(offset)

}

❯ v run bug-offset.v

bug-offset.v:6:28: notice: shifting a value from a signed type `int` can change the sign

    4 |     }

    5 | 

    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`

      |                               ~~~~~~~~~~~~~~~~~~~~~~~

    7 | }

    8 |

bug-offset.v:6:63: notice: shifting a value from a signed type `int` can change the sign

    4 |     }

    5 | 

    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`

      |                                                                  ~~~~~~~~~~~~~~~~~~~~~~~

    7 | }

    8 |

bug-offset.v:6:99: notice: shifting a value from a signed type `int` can change the sign

    4 |     }

    5 | 

    6 |     return bytes[*offset] | ( int(bytes[*offset + 1]) << 8 ) | ( int(bytes[*offset + 2]) << 16 ) | ( int(bytes[*offset + 3]) << 24 ) // error: non-integer index `int`

      |                                                                                                      ~~~~~~~~~~~~~~~~~~~~~~~

    7 | }

    8 |

1

4

So I am not sure why the affectation works ... mistery to me.

It may be related to #17788

I completely forgot about this,thank you, appreciate the explanation (=

[thomas-mangin]

For clarity, if you wanted to use the pattern suggested, the data buffer should also so go into the struct :-)

Still assigning the pointer to another variable should not have changed the type .. so there is probably something to consider here.