How to determine the alignment an object would like
I have a previously allocated chunk of memory that I want to interpret in-place as a struct. How could I determine the memory address within the chunk that has the friendliest alignment for the struct?
Just need to know the mechanism for determining what byte boundary a given struct would work best within, basically.
// psuedo-code
struct Object{
int theseMembersCould;
double beAnything;
char itsJustData[69];
}
// a chunk of previously allocated memory that I want to use
std::vector<uint8> block;
block.resize(1024);
uint32 byteBoundary = ????; // <-- this is what I want to discover
// math to get the nearest addr on the boundary (assumes byteBoundary will be POW2)
uint32 alignmentOffset= (byteBoundary - (block.data() & byteBoundary-1u)) & byteBoundary-1u;
Object * obj = new (block.data() + alignmentOffset) Object;
obj->itsJustData = "used as if it were a normal object beyond this point";
c++ memory memory-alignment
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
add a comment |
I have a previously allocated chunk of memory that I want to interpret in-place as a struct. How could I determine the memory address within the chunk that has the friendliest alignment for the struct?
Just need to know the mechanism for determining what byte boundary a given struct would work best within, basically.
// psuedo-code
struct Object{
int theseMembersCould;
double beAnything;
char itsJustData[69];
}
// a chunk of previously allocated memory that I want to use
std::vector<uint8> block;
block.resize(1024);
uint32 byteBoundary = ????; // <-- this is what I want to discover
// math to get the nearest addr on the boundary (assumes byteBoundary will be POW2)
uint32 alignmentOffset= (byteBoundary - (block.data() & byteBoundary-1u)) & byteBoundary-1u;
Object * obj = new (block.data() + alignmentOffset) Object;
obj->itsJustData = "used as if it were a normal object beyond this point";
c++ memory memory-alignment
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
C++11 and later, look up thealignofoperator,alignasspecifier, andstd::aligned_storagetype trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.
– Peter
Dec 28 '18 at 16:24
add a comment |
I have a previously allocated chunk of memory that I want to interpret in-place as a struct. How could I determine the memory address within the chunk that has the friendliest alignment for the struct?
Just need to know the mechanism for determining what byte boundary a given struct would work best within, basically.
// psuedo-code
struct Object{
int theseMembersCould;
double beAnything;
char itsJustData[69];
}
// a chunk of previously allocated memory that I want to use
std::vector<uint8> block;
block.resize(1024);
uint32 byteBoundary = ????; // <-- this is what I want to discover
// math to get the nearest addr on the boundary (assumes byteBoundary will be POW2)
uint32 alignmentOffset= (byteBoundary - (block.data() & byteBoundary-1u)) & byteBoundary-1u;
Object * obj = new (block.data() + alignmentOffset) Object;
obj->itsJustData = "used as if it were a normal object beyond this point";
c++ memory memory-alignment
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
I have a previously allocated chunk of memory that I want to interpret in-place as a struct. How could I determine the memory address within the chunk that has the friendliest alignment for the struct?
Just need to know the mechanism for determining what byte boundary a given struct would work best within, basically.
// psuedo-code
struct Object{
int theseMembersCould;
double beAnything;
char itsJustData[69];
}
// a chunk of previously allocated memory that I want to use
std::vector<uint8> block;
block.resize(1024);
uint32 byteBoundary = ????; // <-- this is what I want to discover
// math to get the nearest addr on the boundary (assumes byteBoundary will be POW2)
uint32 alignmentOffset= (byteBoundary - (block.data() & byteBoundary-1u)) & byteBoundary-1u;
Object * obj = new (block.data() + alignmentOffset) Object;
obj->itsJustData = "used as if it were a normal object beyond this point";
c++ memory memory-alignment
c++ memory memory-alignment
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
edited Dec 28 '18 at 16:31
Anne Quinn
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
asked Dec 28 '18 at 16:17
Anne QuinnAnne Quinn
4,53563063
4,53563063
C++11 and later, look up thealignofoperator,alignasspecifier, andstd::aligned_storagetype trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.
– Peter
Dec 28 '18 at 16:24
add a comment |
C++11 and later, look up thealignofoperator,alignasspecifier, andstd::aligned_storagetype trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.
– Peter
Dec 28 '18 at 16:24
C++11 and later, look up the
alignof operator, alignas specifier, and std::aligned_storage type trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.– Peter
Dec 28 '18 at 16:24
C++11 and later, look up the
alignof operator, alignas specifier, and std::aligned_storage type trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.– Peter
Dec 28 '18 at 16:24
add a comment |
2 Answers
2
active
oldest
votes
The alignof operator will tell you the alignment required by a type. For example const auto byteBoundary = alignof(Object);.
Consider using std::aligned_storage if you need to create aligned raw memory. You will also need to use placement new to properly being the lifetime of the Object you are trying to use block as.
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by usingnew, your object will not be automatically destroyed. But usingdeletewould also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it withobj->~Object();.
– François Andrieux
Dec 28 '18 at 16:34
add a comment |
Firs off, you attempted use of reinterpret_cast is incorrect, as it leads to undefined behavior due to strict aliasing rule violation. Instead, you should use placement new.
To properly align your struct, you can use std::align together with std::alignof.
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
oldest
votes
active
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active
oldest
votes
The alignof operator will tell you the alignment required by a type. For example const auto byteBoundary = alignof(Object);.
Consider using std::aligned_storage if you need to create aligned raw memory. You will also need to use placement new to properly being the lifetime of the Object you are trying to use block as.
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by usingnew, your object will not be automatically destroyed. But usingdeletewould also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it withobj->~Object();.
– François Andrieux
Dec 28 '18 at 16:34
add a comment |
The alignof operator will tell you the alignment required by a type. For example const auto byteBoundary = alignof(Object);.
Consider using std::aligned_storage if you need to create aligned raw memory. You will also need to use placement new to properly being the lifetime of the Object you are trying to use block as.
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by usingnew, your object will not be automatically destroyed. But usingdeletewould also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it withobj->~Object();.
– François Andrieux
Dec 28 '18 at 16:34
add a comment |
The alignof operator will tell you the alignment required by a type. For example const auto byteBoundary = alignof(Object);.
Consider using std::aligned_storage if you need to create aligned raw memory. You will also need to use placement new to properly being the lifetime of the Object you are trying to use block as.
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
The alignof operator will tell you the alignment required by a type. For example const auto byteBoundary = alignof(Object);.
Consider using std::aligned_storage if you need to create aligned raw memory. You will also need to use placement new to properly being the lifetime of the Object you are trying to use block as.
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
answered Dec 28 '18 at 16:25
François AndrieuxFrançois Andrieux
15.6k32647
15.6k32647
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by usingnew, your object will not be automatically destroyed. But usingdeletewould also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it withobj->~Object();.
– François Andrieux
Dec 28 '18 at 16:34
add a comment |
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by usingnew, your object will not be automatically destroyed. But usingdeletewould also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it withobj->~Object();.
– François Andrieux
Dec 28 '18 at 16:34
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
Ah, I see, changed the code to use placement new
– Anne Quinn
Dec 28 '18 at 16:32
@AnneQuinn Remember that by using
new, your object will not be automatically destroyed. But using delete would also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it with obj->~Object();.– François Andrieux
Dec 28 '18 at 16:34
@AnneQuinn Remember that by using
new, your object will not be automatically destroyed. But using delete would also be problematic, as you are not suppose to release the memory like you would normally. You need to manually call your object's destructor when you are done with it with obj->~Object();.– François Andrieux
Dec 28 '18 at 16:34
add a comment |
Firs off, you attempted use of reinterpret_cast is incorrect, as it leads to undefined behavior due to strict aliasing rule violation. Instead, you should use placement new.
To properly align your struct, you can use std::align together with std::alignof.
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
add a comment |
Firs off, you attempted use of reinterpret_cast is incorrect, as it leads to undefined behavior due to strict aliasing rule violation. Instead, you should use placement new.
To properly align your struct, you can use std::align together with std::alignof.
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
add a comment |
Firs off, you attempted use of reinterpret_cast is incorrect, as it leads to undefined behavior due to strict aliasing rule violation. Instead, you should use placement new.
To properly align your struct, you can use std::align together with std::alignof.
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
Firs off, you attempted use of reinterpret_cast is incorrect, as it leads to undefined behavior due to strict aliasing rule violation. Instead, you should use placement new.
To properly align your struct, you can use std::align together with std::alignof.
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
answered Dec 28 '18 at 16:25
SergeyASergeyA
41.6k53783
41.6k53783
add a comment |
add a comment |
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C++11 and later, look up the
alignofoperator,alignasspecifier, andstd::aligned_storagetype trait. Plenty of information, with examples, on various C++ reference sites. In combination, they can most likely be used to achieve what you need.– Peter
Dec 28 '18 at 16:24