Questions tagged [alignof]
21 questions
72
votes
8 answers
What's the difference between sizeof and alignof?
What's the difference between sizeof and alignof?
#include
#define SIZEOF_ALIGNOF(T) std::cout<< sizeof(T) << '/' << alignof(T) << std::endl
int main(int, char**)
{
SIZEOF_ALIGNOF(unsigned char);
SIZEOF_ALIGNOF(char);
…
user1494506
- 1,283
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49
votes
4 answers
Is it always the case that sizeof(T) >= alignof(T) for all object types T?
For any object type T is it always the case that sizeof(T) is at least as large as alignof(T)?
Intuitively it seems so, since even when you adjust the alignment of objects like:
struct small {
char c;
};
above what it would normally be, their…
BeeOnRope
- 60,350
- 16
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19
votes
3 answers
`std::alignment_of` versus `alignof`
I've just stumbled upon the std::alignment_of type trait, and its soon-to-be friend std::alignment_of_v. They seem to have been specifically designed to be equivalent to a plain call to alignof, and the future addition of the _v helper indicates…
Quentin
- 62,093
- 7
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8
votes
1 answer
Alignment and size of C++ primitive types
In C++, it seems that for all integer types (int, long long int, std::uint16_t, ...) and for the floating point types, it is always sizeof(T) == alignof(T).
Is this compiler/platform-specific, or guaranteed to be true? Could there be a platform…
tmlen
- 8,533
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5
votes
1 answer
How to align a struct member in D?
I tried this
struct Foo(T)
{
align(8) void[T.sizeof] data;
}
but
static assert(Foo!(int).data.alignof == 8);
fails, telling me the alignment is still 1 instead of 8.
Why is this, and how do I fix it, so that it works for any arbitrary…
user541686
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4
votes
1 answer
Writing an alignof(T) method in C#?
Tried implementing alignof(T) formula from this web page but it doesn't match with Unity's:
template
struct alignof
{
enum { s = sizeof (T), value = s ^ (s & (s - 1)) };
};
The actual result, the first sizeof/alignof come from…
aybe
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4
votes
1 answer
Is alignof(T*) is the same for all possible types? What about sizeof(T*)?
Is alignof(T*) is the same value for all possible types T? What about sizeof(T*)?
Please answer based on what is allowed/specified by the standard and not what is the current situation in different compilers.
Koosha
- 1,492
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- 19
4
votes
1 answer
Why is alignof(max_align_t) 16?
I'm running Debian on an x86_64 Intel processor.
gcc (Debian 8.3.0) compiles the following program
#include
#include
#include
int main(){
printf("%zd\n",alignof(max_align_t));
}
and outputs
16
What datatype…
Willis Hershey
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4
votes
1 answer
Why doesn't gcc honor alignment requirements?
Consider the following C++ code that prints the alignment requirement for a double and the effective alignment of a structure member of type double:
#include
struct S { short x; double y; };
int main() {
S s;
std::cout <<…
François Beaune
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3
votes
1 answer
sizeof and types, guarantees
I cannot find a proof/disproof that the following code snippet has no design flaws, speaking about the correctness.
template
class MyDirtyPool {
public:
template
std::size_t add(Args &&... args) {
if…
varnie
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2
votes
1 answer
Effective type in packed raw data
In order to avoid memory fragmentation on very large datasets, I have implemented a doubly linked list that avoid calling malloc twice: one malloc for the data and another one for the prev and next nodes. Instead, it allocates the required space in…
David Ranieri
- 39,972
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2
votes
0 answers
Does `T` always have the same size and alignment as `std::aligned_storage`
Under what circumstances does the standard require that T has exactly the same size and alignment as std::aligned_storage ?
I thought the answer was, always, but from other SO answers today, I learned that for some obscure T,…
Chris Beck
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2
votes
2 answers
How to tell the maximum data alignment requirement in C++
Referencing this question and answer Memory alignment : how to use alignof / alignas? "Alignment is a restriction on which memory positions a value's first byte can be stored." Is there a portable way in C++ to find out the highest granularity of…
Curious
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2
votes
0 answers
When did Clang gain support for C++11's alignof keyword?
Clang first claimed support, via __has_feature(cxx_alignof), for C++11's alignof keyword in r223186 (December 2014). This corresponds to Clang 3.6 by the Clang project's marketing version number.
But that revision only implements the feature-test:…
Jeff Walden
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2
votes
1 answer
How do I fix "The program issued a command but the command length is incorrect." error when calling Process32First()?
GetLastError tells me I'm getting the "The program issued a command but the command length is incorrect." error when calling Process32First() (see code below). I found one post that looked helpful…
Jorge Peach
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