c++ - How to avoid strict aliasing errors when using aligned_storage -


i'm using std::aligned_storage backing storage variant template. problem is, once enable -o2 on gcc start getting warnings of 'dereferencing type-punned pointer break strict aliasing`.

the real template more complex (type checked @ runtime), minimal example generate warning is:

struct foo {   std::aligned_storage<1024> data;    // ... set() uses placement new, stores type information etc ...    template <class t>   t& get()   {     return reinterpret_cast<t&>(data); // warning: breaks strict aliasing rules   } };

i'm pretty sure boost::variant doing same thing this, can't seem find how avoid issue.

my questions are:

  • if using aligned_storage in way violates strict-aliasing, how should using it?
  • is there strict-aliasing problem in get() given there no other pointer based operations in function?
    • what if get() inlined?
    • what get() = 4; get() = 3.2? sequence reordered due int , float being different types?

std::aligned_storage part of <type_traits>; of rest of inhabitants of header file, holder typedefs , not meant used datatype. job take size , alignment, , make pod type characteristics.

you cannot use std::aligned_storage<len, align> directly. must use std::aligned_storage<len, align>::type, transformed type, "a pod type suitable for use uninitialized storage object size @ len , alignment divisor of align." (align defaults largest useful alignment greater or equal len.)

as c++ standard notes, type returned std::aligned_storage array (of specified size) of unsigned char alignment specifier. avoids "no strict aliasing" rule because character type may alias other type.

so might like:

template<typename t> using raw_memory = typename std::aligned_storage<sizeof(t),                                                  std::alignment_of<t>::value>::type;  template<typename t> void* allocate() { return static_cast<void*>(new raw_memory<t>); }  template<typename t, typename ...arg> t* maker(arg&&...arg) {    return new(allocate<t>()) t(std::forward<arg>(arg)...); }

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