Why reference const can be re-assigned in for-statement?

Question

I\'m new to C++, and I\'m confused about this:

vector v = { 1,2 };
const int &r1 = v[0];
//r1 = v[1];  // compiler will show error.
         


        

Answer 1

According to the C++11 standard [stmt.ranged]:

for (const int &r2 : v) std::cout << r2;

where ¹v is a vector, is equivalent to:

{
    auto && __range = (v);
    for (auto __begin = __range.begin(), __end = __range.end(); __begin != __end; ++__begin)
    {
        const int &r2 = *__begin; // <-- new variable in each iteration
        std::cout << r2;
    }
}

Demo

The reference const r2 is assigned twice, right?

No. There is a new r2 variable in each iteration.

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^{¹ The range based for can also be used with other kinds of collections, including raw arrays. The equivalence given here is what the general equivalence becomes for a std::vector.}