CODEWITHSUNDEEP
c++strict-aliasing
user183833
user183833

Reputation: 228

Strict aliasing rule violation

In this link from the isocpp.org faq in the example provided, a Fred object is being constructed with placement new to a buffer that is being allocated for another object i.e. for 

char memory[sizeof(Fred)]

As I know the strict aliasing rules allows us to do the opposite i.e. for an object of whatever type, we are allowed to have a char* point at it and we can dereference that pointer and use it as we want.

But here in the example the opposite is happening. What am I missing?

Upvotes: 2

Views: 231

Answers (2)

M.M
M.M

Reputation: 141648

Placement-new creates a new object. It doesn't alias the old object. The old object (the char array in this example) is considered to stop existing when the placement-new executes.

Before placement-new, there is storage filled with char objects. After placement-new, there is storage filled with one Fred object.

Since there is no aliasing, there are no strict-aliasing problems.

Upvotes: 1

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170239

The strict aliasing rules doesn't mention that Fred* must be cast to char*. Only that variables of type char* and Fred* may point to the same object, and be used to access it.

Quoting [basic.lval] paragraph 8

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

  • the dynamic type of the object,

    [..]

  • a char or unsigned char type.

Upvotes: 2

Related Questions