Why does using reinterpret_cast to convert from char * to structure seem to work fine?

Question

People say that it is not good to trust reinterpret_castto convert from raw data (for example char*) into a structure. For example, for structure

struct A
{
    unsigned int a;
    unsigned int b;
    unsigned char c;
    unsigned int d;
};

sizeof(A) = 16and __alignof(A) = 4as expected.

Suppose I do this:

char *data = new char[sizeof(A) + 1];
A *ptr = reinterpret_cast<A*>(data + 1); // +1 is to ensure it doesn't points to 4-byte aligned data

Then copy some data to ptr:

memcpy_s(sh, sizeof(A),
         "\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04\x00\x00\x00", sizeof(A));

Then ptr->aequals 1, ptr->bequals 2, ptr->cequals 3, and ptr->dequals 4.
Well, it seems to work. Exactly what I expected.

But the data pointed to ptrare not aligned by 4 bytes, for example A. What problems can occur on the x86 or x64 platform? Performance issues?

Answer 1

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Answer 2

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