The alignas type specifier is a portable, C++ standard way to specify custom alignment of variables and user defined types. The alignof operator is likewise a standard, portable way to obtain the alignment of a specified type or variable.

You can use alignas on a class, struck or union, or on individual members. When multiple alignas specifiers are encountered, the compiler will choose the strictest one, (the one with the largest value).

// alignas_alignof.cpp
// compile with: cl /EHsc alignas_alignof.cpp
#include <iostream>

struct alignas(16) Bar  
{      
    int i;       // 4 bytes  
    int n;      // 4 bytes  
    alignas(4) char arr[3];  
    short s;          // 2 bytes  
};  

int main()
{  
    std::cout << alignof(Bar) << std::endl; // output: 16  
}  

Alignment