使用C ++ 11 Range for循环更改字符串中的字符

Question

I read in C++ Primer :

If we want to change the value of the characters in a string, we must define the loop variable as a reference type (§ 2.3.1, p. 50). Remember that a reference is just another name for a given object. When we use a reference as our control variable, that variable is bound to each element in the sequence in turn. Using the reference, we can change the character to which the reference is bound.

Further they give this code :

string s("Hello World!!!");
// convert s to uppercase
for (auto &c : s)   // for every char in s (note: c is a reference)
    c = toupper(c); // c is a reference, so the assignment changes the char
in s
cout << s << endl;

The output of this code is HELLO WORLD!!!

I also read :

There is no way to rebind a reference to refer to a different object . Because there is no way to rebind a reference, references must be initialized.

Question : Won't this code cause rebinding each time the reference variable c is binded to next character of string s ?

for (auto &c : s)   
    c = toupper(c); 

Answer 1

There's no rebinding of an existing variable, at each iteration the "old" c dies and the "new" c is created again, initialized to the next character. That for loop is equivalent to:

{
    auto it = begin(s);
    auto e = end(s);
    // until C++17: auto it = begin(s), e = end(s);
    for(; it!=e; ++it) {
        auto &c = *it;
        c=toupper((unsigned char)c);
    }
}

where you see that, at each iteration, c is re-created and re-initialized.

In other words, a variable declared inside the round parentheses of a range-based for loop has the body of the loop as its scope.

Answer 2

No. A new reference is initialized for each iteration in the for loop.

for (auto &c : s)   
    c = toupper(c); 

is equivalent to:

for (auto it = s.begin(); it != s.end(); ++it)
{
    auto &c = *it;
    c = toupper(c);
}

Answer 3

Consider

char s[5] = {'h','e','l','l','o'};

for (int secret_index=0; secret_index<5; ++secret_index) {
    char &c = s[secret_index];
    c = toupper(c);
}

A new reference (with the same variable name) is initialized on every iteration. That is, the for loop enters and leaves the scope on every iteration.