3,412 questions
3
votes
2
answers
115
views
shortcircuiting logical expressions in preprocessor - actually specified in the C standard?
I'm looking at the C17 standard.
Chapter 6.10, Preprocessing directives
if-group:# if constant-expression new-line groupopt
Chapter 6.5 Expressions->
Chapter 6.6 Constant expressions
11 The ...
- 688
0
votes
1
answer
130
views
Is there a way in Laravel to bulk-insert multiple records and get Eloquent models (all inserted fields) returned? [duplicate]
I have a scenario where I need to insert multiple records at once.
I want to achieve all two:
Bulk insert multiple records at once (performance).
Return Eloquent model instances for the inserted ...
1
vote
1
answer
164
views
Is it standard compliant to declare DTSTART without time zone?
I booked a train ticket with Trenitalia, the Italian train company.
They sent an email with attached ICS files with the travel information.
When I import that ICS in Thunderbird, the time of the train ...
- 5,364
0
votes
2
answers
98
views
How to semantically indicate that a webpage is available in multiple languages
Is there a standard way that an HTML page written in one (human) language can indicate that a translation is available in another language? I know that I can mark up some text manually, saying ...
- 82
3
votes
1
answer
98
views
Why is it not possible to extend a function call expression returning multiple values with another value on the right in Lua
Consider this test case.
function test()
return 1, 2, 3
end
-- OK: prints 4 1 2 3
print (4, test())
-- NOT OK: prints 1 4
print (test(), 4)
Why do values returned from a function in a function ...
- 7,927
5
votes
1
answer
186
views
Why are [[no_unique_address]] members not transparently replaceable?
In the classic talk An (In-)Complete Guide to C++ Object Lifetimes by Jonathan Müller, there is a useful guideline as follows:
Q: When do I need to use std::launder?
A: When you want to re-use the ...
- 44.5k
4
votes
2
answers
183
views
Why is `iterator_category` deleted in `std::views::concat::iterator` if it's a pure input iterator?
In the C++26-adopted proposal p2542, i.e. std::views::concat, there is a confusing statement:
The member typedef-name iterator_category is defined if and only if all-forward<Const, Views...> is ...
- 44.5k
-1
votes
1
answer
164
views
Why is `std::views::as_rvalue` not so cheap as `std::move_iterator`? [closed]
Consider the following code snippet (full code: https://godbolt.org/z/PMM4z9KvM):
int main() {
{
std::cout << "begin std::views::as_rvalue\n";
auto iss = std::...
- 44.5k
2
votes
2
answers
123
views
Why can I not efficiently move the strings when using std::istream_iterator<std::string>?
#include <fstream>
#include <string>
#include <vector>
int main() {
auto fin = std::ifstream("tmp.txt");
auto pos = std::istream_iterator<std::string>(fin);...
- 44.5k
3
votes
1
answer
144
views
Are `asctime_r` and `ctime_r` standard in C?
As we already know, asctime and ctime are not thread-safe since they return a pointer of the internal static variables, cppreference only list the alternatives asctime_s and ctime_s which receive ...
- 4,149
0
votes
1
answer
182
views
Why is a lambda expression not just the syntactic sugar of a functor?
Consider the following code:
int main() {
int n = 0;
return [n] { return n; }(); // ok
}
According to https://cppinsights.io, the code above will be translated into the following code:
int ...
- 44.5k
2
votes
1
answer
131
views
Must T be unqualified in std::allocator<T>?
gcc, clang, and msvc all reject the following code:
#include <memory>
#include <vector>
int main() {
auto _ = std::vector<int const>{}; // error
auto _ = std::vector<...
- 44.5k
1
vote
2
answers
160
views
Why does static_cast not work as the cppref site says in such a case?
Consider the following code:
struct A {
int n;
double d;
};
int main() {
auto a = A{.n = 1, .d = 3.14};
// error: static_cast from 'int*' to 'A*' is not allowed
auto _ = ...
- 44.5k
0
votes
1
answer
193
views
Do we really need std::start_lifetime_as if a well-defined alternative already exists?
Consider the code example excerpted from cppref:
#include <complex>
#include <iostream>
#include <memory>
int main() {
alignas(std::complex<float>) unsigned char buf[...
- 44.5k
0
votes
0
answers
115
views
Does a local constexpr variable occupy the stack space? [duplicate]
constexpr int f(int idx) {
constexpr auto arr = std::array<int, 1'000'000'000>{}; // stack overflow?
return arr[idx];
}
int main() {
return f(1024);
}
Does arr occupy the stack ...
- 44.5k
3
votes
1
answer
223
views
Why is `""s.size()` legal, but `5ms.count()` is not? [duplicate]
#include <chrono>
#include <string>
using namespace std::literals;
int main() {
""s.size(); // ok
(""s).size(); // ok
(5ms).count(); // ok
5ms....
- 44.5k
0
votes
2
answers
134
views
Why can a const object call a non-const member function? [duplicate]
Consider the following code:
int main() {
auto const fn = [] mutable {};
fn(); // ok, why?
}
As per the latest C++ standard draft [expr.prim.lambda.closure] 7.5.6.2/6 (emphasis mine):
The ...
- 44.5k
8
votes
1
answer
459
views
Why is decltype(ConstInt{}) not const?
Consider the following code:
#include <concepts>
#include <string>
using ConstInt = int const;
using ConstStr = std::string const;
static_assert(std::same_as<std::string const, ...
- 44.5k
4
votes
1
answer
242
views
Why were charN_t designed as built-in types, but std::byte was not?
Why were char8_t, char16_t, char32_t designed as built-in types, but std::byte was not?
As per the C++ philosophy, if something can be implemented in the library, we almost always prefer doing so to ...
- 44.5k
3
votes
3
answers
248
views
Does trivially copyable imply trivially relocatable?
C++26 will introduce std::is_trivially_relocatable_v, and the proposal author states: Trivially copyable implies trivially relocatable.
However, I think the statement might not always be true, ...
- 44.5k
2
votes
1
answer
117
views
Why do we keep a redundant ctor in std::copyable_function?
According to the C++ docs, std::copyable_function has two overloaded ctors as follows:
template<class T, class... CArgs>
explicit copyable_function(std::in_place_type_t<T>,
CArgs&&...
- 44.5k
3
votes
1
answer
186
views
Why does std::function_ref allow passing in an expiring functor, rather than disallow it?
At https://en.cppreference.com/w/cpp/utility/functional/function_ref/function_ref.html, there is an overloaded ctor as follows:
template<class F>
function_ref(F&& f) noexcept;
...
- 44.5k
0
votes
1
answer
56
views
RDF standard ontology for statement with a start and end
I am building an application that builds SPARQL query, and I'm trying to make it as generic as possible.
In my data, I need to be able to model some affiliations between people and structures. These ...
- 89
-1
votes
1
answer
86
views
Using true/false vs 1/0 in MongoDB Projections — Long-Term Compatibility [closed]
In our Go codebase, we’re building dynamic projection maps for queries. Instead of using map[string]int{"name": 1}, we’re considering using map[string]bool{"name": true} for ...
0
votes
0
answers
107
views
Why does std::equality_comparable_with not work in the simplest case? [duplicate]
#include <concepts>
struct A {};
struct B {};
constexpr bool operator==(A, A) {
return true;
}
constexpr bool operator==(B, B) {
return true;
}
constexpr bool operator==(A, B) {
...
- 44.5k
1
vote
1
answer
114
views
Does the IOC (Implicitly Object Creation) apply to my_alloc wrapping std::malloc?
Consider p0593 and the following code:
struct A { std::string s; };
static_assert(std::is_implicit_lifetime_v<A>); // true!
// std::malloc is explicitly BLESSED by the C++23 standard.
// But ...
- 44.5k
2
votes
1
answer
294
views
Why is it UB to pass a char array as the argument of placement new?
I think the following code is ubiquitous:
struct A { ... };
char buf[1024];
auto ptr = new (buf) A(...);
However, new (buf) A(...); is UB. https://en.cppreference.com/w/cpp/language/lifetime.html ...
- 44.5k
3
votes
1
answer
151
views
Why add redundant `std::constructible_from<T>` to the concept `std::default_initializable`?
At the cppref page on the concept std::default_initializable, I saw the following code:
template<class T>
concept default_initializable =
std::constructible_from<T> && ...
- 44.5k
3
votes
1
answer
95
views
Is it allowed in javascript to modify the handler after proxy has already been created?
I modify the handler object after proxy's creation, and obtain intended result. But nowhere in mdn document such pattern, can i rely on this pattern to work consistently across browser/in future?
...
- 89
5
votes
2
answers
288
views
Why was this sentence removed from C23?
I usually referred to C17, but decided to switch to C23.
And looking through the C23 standard, I saw that the sentence from C17 was deleted in C23:
A declaration other than a static_assert ...
- 639
1
vote
1
answer
77
views
Do I need to set a CL_MEM_READ_WRITE when creating a buffer?
When creating a buffer in OpenCL, one passes a flags bitfield. One of these possible flags is CL_MEM_READ_WRITE, being the lowest bit in the field (value 1 << 0). Its documentation says that &...
- 137k
1
vote
1
answer
113
views
is using the + operator a way to remove qualifiers within the typeof operator instead of using typeof_unqual in C23?
C23 added the typeof_unqual keyword as a way to get the unqualified type of an object. Can't you just use the + operator on scalar types to avoid this? (I know this doesn't work on struct/union typed ...
- 1,025
4
votes
1
answer
112
views
Why does C++ allow std::function assignments to imply an object copy?
My question concerns the line of code below marked with the // WHY??? comment.
Below is a toy example of some code I was expecting to break: a std::function variable is assigned a function pointer ...
- 3,361
1
vote
0
answers
29
views
How should email addresses that contain / be encoded in mailto: URLs?
Which of the following should be the URL for the email address ~a/[email protected]?
mailto:~a/[email protected]
mailto:~a%[email protected]
Although most clients are able to handle (1), is (2) required by ...
- 412
21
votes
1
answer
1k
views
Did C++20 change to allow conversion from array-of-known-bound `type(&)[N]` to array-of-unknown-bound `type(&)[]`?
Context:
In C++, it's possible to create a reference to an "array of unknown bound".
Example: const char (&)[]
Before C++20, clang (14.x, 18.x) did not allow you to coerce a reference to ...
- 19.8k
0
votes
0
answers
60
views
Is integer overflow defined behavior in Fortran? [duplicate]
According to the C standard, signed integer overflow is undefined behavior. Fortran integers are signed integers. Does the Fortran standard mandate how overflow should be treated? In practice it seems ...
- 236
0
votes
0
answers
33
views
PayPal Standard Checkout is not generating an OAuth token (PHP) [duplicate]
I'm trying to implement PayPal Standard Checkout in PHP, and when I make a server-side request, this error appears:
"Following authentication credentials are required:\n-> Client is not ...
0
votes
0
answers
121
views
Why did Stroustrup write in PPP (2nd editioln) that operator ++ does not work with type char?
I've read in Stroustrup PPP "Programming Principles and Practice Using C++", 2nd edition in table "useful operators for some common and useful types" here
that type "char&...
- 19
3
votes
1
answer
121
views
Why is a consteval function expression NOT a primary expression?
Consider the following code snippet (compiled with clang19):
consteval bool IsEven(int n) {
return 0 == n % 2;
}
template<int n>
requires (IsEven(n)) // ok
void f1() {}
template<int ...
- 44.5k
2
votes
0
answers
117
views
Sizeof equality of direct members vs inherited members
Does the C++ standard have anything to say about the size equality of the following?
struct AB
{
int a;
int b;
}
Or
struct A
{
int a
};
struct B : public A
{
int b;
}
Is sizeof(...
- 1,489
16
votes
1
answer
950
views
Is `std::function` deprecated by `std::copyable_function` in C++26?
C++26 provides std::copyable_function [cppref link]. However, the existing std::function is already copyable. So, I have 3 questions:
What are the key advantages of std::copyable_function over std::...
- 44.5k
13
votes
1
answer
144
views
Why use `(void)++p` rather than just `++p` in this C++ standard proposal? [duplicate]
In a C++ standard proposal, I find a strange code snippet as follows:
template <input_iterator I, sentinel_for<I> S, nothrow_forward_iterator I2>
constexpr auto uninitialized_copy(I first, ...
- 44.5k
0
votes
1
answer
68
views
What are the Standards and The Best Practice In c? [closed]
i'm new here, i'm trying learn c and also i want to know the best practices to write good quality code and legible code, Thanks.
i'm tried search the standards and best practices in c but i didn't ...
- 11
3
votes
1
answer
303
views
Is shifting 0 to the left considered undefined behavior in C?
Is either of these expressions Undefined Behavior (UB)?
0 << 32
0LL << 64
I would appreciate it if the answer includes references to the C standard.
- 531
0
votes
1
answer
74
views
Standard Method of Supporting Vendors Using C++ and Premake
I am working on a template that makes use of Premake and GoogleTest, but I am having issues finding any documentation on a standardized method of implementing Premake. The source that I have been ...
3
votes
1
answer
172
views
C standard - const qualification of struct members
I encountered a problem at work today regarding const propagation in structs.
I was trying to add some const correctness to old code when I ran into an issue where two different compilers disagreed, ...
- 41
0
votes
1
answer
63
views
Difference between chr and Char.chr in Standard ML
My program:
val a = chr 65 : char
val b = Char.chr 65: char
val _ = print (Char.toString a)
val _ = print (Char.toString b)
Output:
AA
Lines 1 and 2 show that both chr and Char.chr work.
Is there a ...
- 21.2k
5
votes
2
answers
210
views
Regarding POSIX ERE's new ? modifier with nested repetitions
The 2024 version of POSIX specifies the ? repetition modifier. In the published document, it's specified that the ? modifier changes the behavior of its immediately preceeding repetition from matching ...
- 1,671
1
vote
0
answers
72
views
MIDI spec: "respond accordingly" to missing LSB or MSB of parameter number
What does the MIDI 1.0 Detailed Specification mean by "respond accordingly" to receiving only a least-significant byte (LSB) or a most-significant byte (MSB) (but not both) for a (non-)...
- 2,946
3
votes
0
answers
156
views
Did ISO C++ make any attempt to standardize `unsafe {}` blocks/functions/namespaces in the language?
The unsafe keyword is a relatively familiar language feature, known mostly from C# and Rust.
The general idea of the feature is to define "safe" subset and then require demarking any usage ...
- 29.9k