Question

suppose a struct defined like this:

struct S{
    char a[3];
    char b[3];
    char c[3];
};

then what will be the output of printf(“%d”, sizeof(S)) ? On My compiler of Vc++ 2008 expression, the output is 9. And I got confused… I suppose the result be 12, but it is not. Shouldn’t the compiler align the structure to 4 or 8 ?

Answer 1

The value of the sizeof-expression is implementation-dependent; the only thing guaranteed by the C++ standard is that it must be at least nine since you’re storing nine char‘s in the struct.

The new C++11 standard has an alignas keyword, but this may not be implemented in VC++08. Check your compiler’s manual (see e.g. __declspec(align(#))).