&string interpolation with += string

[Eliyaan]

V version: V 0.4.9 a20fff9, press to see full `v doctor` output

V full version	V 0.4.9 6ac1d08.a20fff9
OS	linux, "Void Linux"
Processor	4 cpus, 64bit, little endian, Intel(R) Pentium(R) Gold 7505 @ 2.00GHz
Memory	16.67GB/23.16GB
V executable	/home/nopana/v/v
V last modified time	2025-01-13 21:21:06
V home dir	OK, value: /home/nopana/v
VMODULES	OK, value: /home/nopana/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/nopana/projects/tests
Git version	git version 2.47.1
V git status	weekly.2025.1-44-ga20fff99
.git/config present	true
cc version	cc (GCC) 13.2.0
gcc version	gcc (GCC) 13.2.0
clang version	N/A
tcc version	tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 0134e9b9
emcc version	N/A
glibc version	ldd (GNU libc) 2.39

What did you do?
./v -g -o vdbg cmd/v && ./vdbg bug.v && bug

mut s := "Hello"
mut p := &s
p += s

What did you see?

bug.v:3:3: warning: pointer arithmetic is only allowed in `unsafe` blocks
    1 | mut s := "Hello"
    2 | mut p := &s
    3 | p += s
      |   ~~
bug.v:2:5: warning: unused variable: `p`
    1 | mut s := "Hello"
    2 | mut p := &s
      |     ^
    3 | p += s
bug.v:3:3: error: mismatched types `&string` and `string`
    1 | mut s := "Hello"
    2 | mut p := &s
    3 | p += s
      |   ~~
bug.v:3:1: error: operator `+=` not defined on left operand type `string`
    1 | mut s := "Hello"
    2 | mut p := &s
    3 | p += s
      | ^

What did you expect to see?

To say it is not defined on &string (because it is defined for string). But I dont know if it should allow for &string += string .

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[huly-for-github]

Connected to Huly®: V_0.6-21890

[JalonSolov]

What did you expect to happen with your example? Looks you're trying to add a string to a string pointer, which is exactly what V said.

[jorgeluismireles]

This works even without mut in the playground:

fn main() {
	s := "Hello"
	p := &s
	unsafe { *p += s }
	println('${*p}') // prints HelloHello
}

[jorgeluismireles]

Exponential grow without mut:

fn main() {
	s := "Hello"
	p := &s
	for i := 0; i < 4; i++ {
		unsafe { *p += *p }
	}
	println('${s}') // Hello 2^4 = 16 times
}

[Eliyaan]

I guess I did not understand properly what V meant by "pointer arithmetic".
I wanted to concatenate a string to the string stored in the reference

Also it says that += is not defined on type string, instead of &string

[JalonSolov]

No, += is not defined for adding a string to a pointer.

And adding a string to the string pointed to by a pointer will just overwrite unallocated memory (or worse, allocated for something else). Both of @jorgeluismireles options may "work", but they have corrupted memory beyond what was correctly allocated for s.

The string struct also has not been properly updated, so s.len will be incorrect, and when it is free'd, it will only free what was originally allocated for s, not the extra that was crammed onto the end through pointers. Valgrind will certainly complain about the leaked memory, supposing your code doesn't crash the machine (depending on what gets overwritten).

So, although it is certainly possible to make this kind of thing work, it is definitely unsafe, and should never be used... at least without a lot more code added to make sure memory is properly allocated, and the struct for s properly updated.

[jorgeluismireles]

V pointer arithmetic message seems to be like the C pointer arithmetic, but seems is not. In C, with pointer arithmetic you can read and write each byte of an integer, struct, string, array... having the start of the object. But you are in your own because you can modify parts of other objects adjacent set by the compiler and create unexpected bugs or create viruses or security threats. In V seems to be just the possibility of use the += operator, but as is marked you are not safe.

[Eliyaan]

Okay thank you!