offsetof

offsetof std::size_t, , , .

o type , o.member lvalue, o. . , member , -, .

type PODType ( C++11) ( C++11), offsetof ( C++17) offsetof ( C++17).

offsetof(type, member) , type .

offsetof .

offsetof C++ : GCC, LLVM.

member . , . C DR 496.

C23 , offsetof , C++: offsetof(struct Foo { int a; }, a) ICC GCC, offsetof(struct Foo { int a, b; }, a) - Foo.

#include <cstddef>
#include <iostream>

struct S
{
    char   m0;
    double m1;
    short  m2;
    char   m3;
//  private: int z; // : 'S'    
};

int main()
{
    std::cout
        << " char   m0 = " << offsetof(S, m0) << '\n'
        << " double m1 = " << offsetof(S, m1) << '\n'
        << " short  m2 = " << offsetof(S, m2) << '\n'
        << " char   m3 = " << offsetof(S, m3) << '\n';
}

 char   m0 = 0
 double m1 = 8
 short  m2 = 16
 char   m3 = 18

C++: