modern_cpp_dsa.md

Data structure alignment (alignas, alignof)

---

alignas keyword

The alignas specifier may be applied to:

• the declaration of variable or a class data member
• the declaration or definition of a class/struct/union or enumeration

alignas(expression) - expression needs to be positive power of 2.

alignas(type-id) - equivalent to alignas(alignof(type-id))

alignas(0) - has no effect

Exception: if alignas would weaken the alignment the type have had without this alignas, it will not be applied.

---

alignas keyword

// every object of type see_t will be alignet to 16-byte boundary
struct alignas(16) see_t {
    float see_data[4];
};

// error: requested alignment is not a positive power of 2 alignas(129) char cacheline[128];
alignas(129) char cacheline[128];

---

alignof keyword

The alignof specifier returns a value of type std::size_t, which is alignment in bytes. If the type is reference type, the operator returns the alignment of referenced type; if the type is array type, alignment requirement of the element type is returned.

#include <iostream>
using namespace std;

struct Foo {
    int i;
    float f;
    char c;
};
struct Empty{};
struct alignas(64) Empty64 {};
struct alignas(1) Double {
    double d;
};

int main() {
    cout << "Alignment of" << '\n'
         << "char: "      << alignof(char)    << '\n' // 1
         << "pointer: "   << alignof(int*)    << '\n' // 8
         << "class Foo: " << alignof(Foo)     << '\n' // 4
         << "Empty: "     << alignof(Empty)   << '\n' // 1
         << "Empty64: "   << alignof(Empty64) << '\n' // 64
         << "Double: "    << alignof(Double)  << '\n' // 8
}

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Exercise

Change the alignment of the Circle class to 128.

Print the alignment in main() function.

Change the alignment to 2.

Print the alignment.