std::launder and strict aliasing rule

问题

Consider this code:

void f(char * ptr)
{
    auto int_ptr = reinterpret_cast<int*>(ptr); // <---- line of interest 
    // use int_ptr ... 
}

void example_1()
{
    int i = 10;    
    f(reinterpret_cast<char*>(&i));
}

void example_2()
{
    alignas(alignof(int)) char storage[sizeof(int)];
    new (&storage) int;
    f(storage);
}

line of interest with call from example_1:

Q1: On the callside the char pointer is aliasing our integer pointer. This is valid. But is it also valid to just cast it back to an int? We know an int is within its lifetime there, but consider the function is defined in another translation unit (with no linktime optimization enabled) and the context is not known. Then all the compiler sees is: an int pointer wants to alias a char pointer, and this is violating the strict aliasing rules. So is it allowed?

Q2: Considering its not allowed. We got std::launder in C++17. Its kind of a pointer optimization barrier mostly used to access an object which got placement new'ed into the storage of an object of other type or when const members are involved. Can we use it to give the compiler a hint and prevent undefined behavior?

line of interest with call from example_2:

Q3: Here std::launder should be required, since this is the std::launder use case described in Q2, right?

auto int_ptr = std::launder(reinterpret_cast<int*>(ptr));

But consider again f is defined in another translation unit. How can the compiler know about our placement new, which happens on the callside? How can the compiler (only seeing function f) distinguish between example_1 and example_2? Or is all above just hypotetical, since the strict aliasing rule would just ruling out everything (remember char* to int* not allowed) and the compiler can do what he wants?

Follow up question:

Q4: If all code above is wrong due to aliasing rules, consider changing the function f to take a void pointer:

void f(void* ptr)
{
    auto int_ptr = reinterpret_cast<int*>(ptr); 
    // use int_ptr ... 
}

Then we have no aliasing problem, but still there is the std::launder case for example_2. Do we have change the callside and rewrite our example_2 function to:

void example_2()
{
    alignas(alignof(int)) char storage[sizeof(int)];
    new (&storage) int;
    f(std::launder(storage));
}

or is std::launder in function f sufficient?

回答1:

The strict aliasing rule is a restriction on the type of the glvalue actually used to access an object. All that matters for the purpose of that rule are a) the actual type of the object, and b) the type of the glvalue used for the access.

The intermediate casts the pointer travels through are irrelevant, as long as they preserve the pointer value. (This goes both ways; no amount of clever casts - or laundering, for that matter - will cure a strict aliasing violation.)

f is valid as long as ptr actually points to an object of type int, assuming that it accesses that object via int_ptr without further casting.

example_1 is valid as written; the reinterpret_casts do not change the pointer value.

example_2 is invalid because it gives f a pointer that doesn't actually point to an int object (it points to the out-of-lifetime first element of the storage array). See Is there a (semantic) difference between the return value of placement new and the casted value of its operand?

来源：https://stackoverflow.com/questions/51204362/stdlaunder-and-strict-aliasing-rule