Questions tagged [bit-cast]
14 questions
14
votes
1 answer
Is it valid to create closure (lambda) objects using `std::bit_cast` in C++20?
A colleague showed me a C++20 program where a closure object is virtually created using std::bit_cast from the value that it captures:
#include
#include
class A {
int v;
public:
A(int u) : v(u) {}
auto getter() const {…
Fedor
- 17,146
- 13
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9
votes
0 answers
Why can't I std::bit_cast the contents of a string literal?
While experimenting with compile-time string manipulation, I've encountered a strange phenomenon:
#include
constexpr const char str[4] = "abc";
// error: constexpr variable 'x' must be initialized by a constant expression
constexpr auto x =…
Jan Schultke
- 17,446
- 6
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- 96
3
votes
2 answers
Trying to use std::bit_cast with a bitfield struct. Why is it not constexpr?
I'm trying to get a bitmask from a bitfield struct, at compile time. One of the tricks that I tried, which looks promising to me, is using std::bit_cast, because it is supposed to be constexpr.
My test code can be found here:…
sh-
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3
votes
1 answer
Can bit_cast do the same thing as memcpy to store a pointer into an array of shorts without undefined behaviour?
While there might be an XY issue that led me here, I am curious about the limits of the new bit_cast in C++20, and this seems a good illustration of it.
Note that there is nothing inherently wrong with std::memcpy and it does not cause undefined…
Glenn Teitelbaum
- 10,108
- 3
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3
votes
2 answers
Safe equivalent of std::bit_cast in C++11
C++20 introduced std::bit_cast for treating the same bits as if they were a different type. So, basically it does this:
template
T2 undefined_bit_cast(T1 v1) {
T1 *p1 = &v1;
T2 *p2 = (T2 *)p1; // Uh oh.
T2 v2 =…
Craig Gidney
- 17,763
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- 136
1
vote
2 answers
Bitwise comparing NaN's in C++
I have a function that returns a double. Any real number is a valid output. I'm using nan's to signal errors. I am error checking this way.
double foo();
const auto error1 = std::nan("1");
const auto error2 = std::nan("2");
const auto error3 =…
Estin
- 13
- 3
1
vote
1 answer
Syntax for std::bit_cast to an array
#include
#include
struct A {
int a[100];
};
struct B {
short b[200];
};
void test(const A &in) {
const auto x = std::bit_cast(in); // error: no matching function for call to 'bit_cast(const…
Home of the Brave
- 133
- 8
1
vote
1 answer
std::bit_cast vs reinterpret_cast in file I/O
How should one cast pointers to char*/const char*? Using reinterpret_cast? Or probably std::bit_cast?
A simple example:
#include
#include
#include
int main( )
{
std::uint32_t var { 301 };
{
std::ofstream…
digito_evo
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1
vote
1 answer
How can I convert ANY user defined type to an std::bitset?
What I want to achieve is a means of converting any arbitrarily sized and formatted type to an std::bitset. Like this:
#include
#include
#include
#include
#include
#include
template
dave_thenerd
- 448
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- 10
0
votes
1 answer
Design choices for writing efficient numerical solvers in c++: Type punning
I'm writing a numerical fluid solver in C++ as a hobby project. I will try to explain what I want to accomplish in a simplified manner.
The solver has multiple flow variables (density, velocity, pressure, etc.) stored in each cell in a grid. I would…
ander
- 11
- 3
0
votes
0 answers
std::bit_cast to pointer type not useable in constexpr context
Reading thru https://github.com/isocpp/CppCoreGuidelines/issues/1517 I got the idea to get rid of the non-constexpr reinterpret_cast from my µC code (e.g. C could be a µC internal peripheral on address 0x100). Therefore I tried std::bit_cast-ing to…
wimalopaan
- 4,838
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0
votes
1 answer
How to let gcc optimize std::bit_cast with std::move?
Consider below code:
#include
#include
#include
struct A { uint32_t a[100]; };
struct B { uint16_t b[200]; };
void test(const A&);
void foo() {
B tmp;
test(std::bit_cast(std::move(tmp)));
}
void bar() {
B…
Home of the Brave
- 133
- 8
0
votes
0 answers
Does reinterpret_cast together with binary input/output always leads to undefined behavior?
When working binary input/output, we often see/write code similar to this
in_file.read(reinterpret_cast(&obj), sizeof(obj))
out_file.write(reinterpret_cast(&obj), sizeof(obj))
Now, is there any guarantee within the standard…
Myrddin Krustowski
- 701
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0
votes
1 answer
What, if any, additional UB will bit_cast have beyond memcpy?
I have a class whose purpose is to move data which might have alignment restrictions to or from a serialized memory buffer where the data is not aligned. I have set and get handlers as follows:
#include
template
struct…
markt1964
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