Why can we not access elements of a tuple by index?

Question

tuple <int, string, int> x=make_tuple(1, "anukul", 100); cout << x[0];       //1 cout << get<0>(x);  //2 

2 works. 1 does not.

Why is it so?

From Lounge C++ I learnt that it is probably because the compiler does not know what data type is stored at that index. But it did not make much sense to me as the compiler could just look up the declaration of that tuple and determine the data type or do whatever else is done while accessing other data structures' elements by index.

Answer 1

Because [] is an operator (named operator[]), thus a member function, and is called at run-time.

Whereas getting the tuple item is a template mechanism, it must be resolved at compile time. Which means this can be only done with the <> templating syntax.

To better understand, a tuple may store different types. A template function may return different types depending on the index passed, as this is resolved at compile time. The operator[] must return a unique type, whatever the value of the passed parameter is. Thus the tuple functionality is not achievable.

get<0>(x) and get<1>(x) are two different functions generated at compile time, and return different types. The compiler generates in fact two functions which will be mangled to something like

int get_tuple_int_string_int_0(x) 

and

string get_tuple_int_string_int_1(x)