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りき萌 2024-07-22 20:34:42 +02:00
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commit 2a68cfbf63
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22
content/programming/blog/lvalues.tree
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@ -5,6 +5,7 @@
% id = "01HY5R1ZV9G92SVA0XP7CG1X6K"
- as in this case:
```c
void example(int *x) {
int y = *x; // (1)
@ -19,10 +20,10 @@
- doesn't `*x` mean "read the value pointed to by `x`?"
% id = "01HY5R1ZV9WJM9DMW5QGK04DCR"
- TL;DR: the deal with this example is that `*x` *does not mean* "read from the location pointed to by `x`", but _just_ "the location pointed to by `x`".
- TL;DR: the deal with this example is that `*x` _does not mean_ "read from the location pointed to by `x`", but _just_ "the location pointed to by `x`".
% id = "01HY5R1ZV9JN9GZ8BECJ0KV1FC"
- **`*x` is not a _value_, it's a _memory location_, or _place_ in Rust parlance**
- *`*x` is not a _value_, it's a _memory location_, or _place_ in Rust parlance*
% id = "01HY5R1ZV9QVH3W5BRS16V6EPG"
- same thing with `x`
@ -154,6 +155,7 @@ for starters, it is impossible to write the type down in C code, so we're always
% id = "01HY5R1ZV9VZFHRKY4QWBDTFJ7"
- example:
```c.types
void example(void) {
struct S { int x; } s;
@ -168,6 +170,7 @@ for starters, it is impossible to write the type down in C code, so we're always
the function signature is therefore `T* -> ptrdiff_t -> place(T)`.
example:
```c.types
void example(int* array) {
int* p = &((array /*: int* */)[123] /*: place(int) */);
@ -184,7 +187,7 @@ for starters, it is impossible to write the type down in C code, so we're always
% id = "01HY5R1ZV9MQWK5VQZQG8JJ03J"
- I do wonder though why it doesn't produce a warning.
I'm no standards lawyer, but I *believe* this may have something to do with implicit type conversions - the `0` gets promoted to a
I'm no standards lawyer, but I _believe_ this may have something to do with implicit type conversions - the `0` gets promoted to a
pointer as part of the desugared addition.
I really need to read up about C's type promotion rules.
@ -193,8 +196,8 @@ for starters, it is impossible to write the type down in C code, so we're always
there are no places in C.
% id = "01HY5R1ZV9PAD1XRS29YV64GV2"
- the C standard actually calls this concept "lvalues", which comes from the fact that they are **values** which are valid
**l**eft-hand sides of assignment.
- the C standard actually calls this concept "lvalues", which comes from the fact that they are *values* which are valid
*l*eft-hand sides of assignment.
% id = "01HY5R1ZV9BNZM45RK2AW8BF5N"
+ however, I don't like that name since it's quite esoteric - if you tell a beginner "`x` is not an lvalue," they will look at you confused.
@ -223,7 +226,8 @@ there are no places in C.
in layman's terms, C++ makes it impossible to rebind references to something else.
you can't make this variable point to `y`:
<!-- NOTE: using `c` syntax here instead of `cpp` because I don't have a working C++ syntax at the moment! -->
{% NOTE: using `c` syntax here instead of `cpp` because I don't have a working C++ syntax at the moment! %}
```c
int x = 0;
int y = 1;
@ -253,10 +257,11 @@ there are no places in C.
- I actually kind of wish references were more like they are in Rust - basically just pointers but non-null and guaranteed to be aligne
% id = "01HY5R1ZV9QWHZRJ5V53CVYG5V"
- anyways, as a final ~~boss~~ bonus of this blog post, I'd like to introduce you to the `x->y` operator (the C one)
- anyways, as a final {-boss-} bonus of this blog post, I'd like to introduce you to the `x->y` operator (the C one)
% id = "01HY5R1ZV9AYFWV96FC07WX68G"
- if you've been programmming C or C++ for a while, you'll know that it's pretty dangerous to just go pointer-[walkin'](https://www.youtube.com/watch?v=d_dLIy2gQGU) with the `->` operator
```c
int* third(struct list* first) {
return &list->next->next->value;
@ -278,6 +283,7 @@ there are no places in C.
% id = "01HY5R1ZV9X8M5T64BD0MZ2WN2"
- let's start by dismantling the entire pointer access chain into separate expressions:
```c
int* third(struct list* first) {
struct list* second = first->next;
@ -288,6 +294,7 @@ there are no places in C.
% id = "01HY5R1ZV9PGQMS2A6H5XTWYZX"
- now let's desugar the `->` operator:
```c
int* third(struct list* first) {
struct list* second = (*first).next;
@ -298,6 +305,7 @@ there are no places in C.
% id = "01HY5R1ZV92CZAV13P4KFABTR1"
- and add some type annotations:
```c.types
int* third(struct list* first) {
struct list* second = (*first).next /*: place(struct list*) */;