In C++ placement new is used to construct an object at a particular memory location or to pass additional arguments to an allocation function.
Questions tagged [placement-new]
391 questions
27
votes
5 answers
Do I really have to worry about alignment when using placement new operator?
I read this When should I worry about alignment? but I am still do not know if I have to worry about not aligned pointer returned by placement new operator - like in this example:
class A {
public:
long double a;
long long b;
A() : a(1.3),…
PiotrNycz
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22
votes
2 answers
Is it allowed to call destructor explicitly followed by placement new on a variable with fixed lifetime?
I know that calling destructor explicitly can lead to undefined behavior because of double destructor calling, like here:
#include
int main() {
std::vector foo(10);
foo.~vector();
return 0; // Oops, destructor will be…
yeputons
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22
votes
7 answers
Using new (this) to reuse constructors
This came up recently in a class for which I am a teaching assistant. We were teaching the students how to do copy constructors in c++, and the students who were originally taught java asked if you can call one constructor from another. I know the…
Brandon Bodnar
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20
votes
4 answers
How to delete object constructed via placement new operator?
char * buf = new char[sizeof(T)];
new (buf) T;
T * t = (T *)buf;
//code...
//here I should destruct *t but as it is argument of template and can be
//instantiated via basic types as well (say int) so such code
/*t->~T();*/
//is incorrect (maybe…
Mihran Hovsepyan
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Is it safe to call placement new on `this` for trivial object?
I know that this question was asked several times already but I couldn't find an answer for this particular case.
Let's say I have a trivial class that doesn't own any resources and has empty destructor and default constructor. It has a handful of…
Amomum
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19
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2 answers
Why is this code trying to call the copy constructor?
I just spent an inordinate amount of time fiddling with a complilation error in Visual Studio. I have distilled the code into the small compilable example below and tried it on IdeOne and got the same error which you can see here.
I am wondering why…
Seth Carnegie
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19
votes
3 answers
Is placement new legally required for putting an int into a char array?
There seems to be some agreement that you can't willy nilly point (an int*) into a char array because of the C++ aliasing rules.
From this other question -- Generic char[] based storage and avoiding strict-aliasing related UB -- it seems that it is…
Martin Ba
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18
votes
3 answers
Can placement-new and vector::data() be used to replace elements in a vector?
There are two existing questions about replacing vector elements that are not assignable:
C++ Use Unassignable Objects in Vector
How to push_back without operator=() for const members?
A typical reason for an object to be non-assignable is that…
jogojapan
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17
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5 answers
Mixing operator new[] and placement new with ordinary delete[]
Just out of curiosity, is the following legal?
X* p = static_cast(operator new[](3 * sizeof(X)));
new(p + 0) X();
new(p + 1) X();
new(p + 2) X();
delete[] p; // Am I allowed to use delete[] here? Or is it undefined behavior?
Similarly:
X* q…
fredoverflow
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17
votes
3 answers
C++ Volatile Placement New
How does one do a placement new operation on a volatile pointer.
For example, I want to do something like this:
volatile SomeStruct Object;
volatile SomeStruct* thing = &Object;
new (thing) SomeStruct(/*arguments to SomeStruct's constructor*/);
I…
DarthRubik
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4 answers
CUDA: Wrapping device memory allocation in C++
I'm starting to use CUDA at the moment and have to admit that I'm a bit disappointed with the C API. I understand the reasons for choosing C but had the language been based on C++ instead, several aspects would have been a lot simpler, e.g. device…
Konrad Rudolph
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17
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1 answer
'operator new' : function does not take 2 arguments
I cannot seem to get my placement new to work for some reason. Based on this question, I have set this up correctly.
However, I continue to get the error:
'operator new' : function does not take 2 arguments
Here is my code:
char * p = new char…
Serguei Fedorov
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16
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2 answers
std::optional implemented as union vs char[]/aligned_storage
While reading through GCC's implementation of std::optional I noticed something interesting. I know boost::optional is implemented as follows:
template
class optional {
// ...
private:
bool has_value_;
aligned_storage
Ron
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16
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Is there a (semantic) difference between the return value of placement new and the casted value of its operand?
Is there a (semantic) difference between the return value of placement new and the casted value of its operand?
struct Foo { ... };
char buffer[...];
Foo *a = new(buffer) Foo;
Foo *b = reinterpret_cast(buffer);
Does a and b differ in some…
geza
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16
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3 answers
How C++ placement new works?
This question is to confirm I understood the concept right and take expert opinion on the style of usages and possible optimization.
I am trying to understand "placement new" and following is the program I came up with...
#include
…
vikrant
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