Is it legal to index into a struct?

Question

Regardless of how \'bad\' the code is, and assuming that alignment etc are not an issue on the compiler/platform, is this undefined or broken behavior?

If I have a s

Answer 1

In C++ if you really need it - create operator[]:

struct data
{
    int a, b, c;
    int &operator[]( size_t idx ) {
        switch( idx ) {
            case 0 : return a;
            case 1 : return b;
            case 2 : return c;
            default: throw std::runtime_error( "bad index" );
        }
    }
};


data d;
d[0] = 123; // assign 123 to data.a

it is not only guaranteed to work but usage is simpler, you do not need to write unreadable expression (&thing.a)[0]

Note: this answer is given in assumption that you already have a structure with fields, and you need to add access via index. If speed is an issue and you can change the structure this could be more effective:

struct data 
{
     int array[3];
     int &a = array[0];
     int &b = array[1];
     int &c = array[2];
};

This solution would change size of structure so you can use methods as well:

struct data 
{
     int array[3];
     int &a() { return array[0]; }
     int &b() { return array[1]; }
     int &c() { return array[2]; }
};