auto reference to address in C-CodePudding

I'm studying some C features, trying to play around with some experiments. However, I stuck in a place where it compiled error:

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string str = "some string";
    auto &c = str.begin(); // compile error
    *c = toupper(*c);
    cout << *c << ", str: " << str << endl;
}

I'm not sure why it was not acceptable. My thought was that c had type char * (a pointer to a char), so that's why I had written as above. But why it failed in compiling?

Error C2440 Cannot transform 'std::_String_iteratorstd::_String_val<std::_Simple_types<_Elem>>' to'std::_String_iterator<std::_String_val<std::_Simple_types<_Elem

PS: Another method which I had tried first was OK.

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string str = "some string";
    auto &c = *str.begin(); // success
    c = toupper(c);
    cout << c << ", str: " << str << endl;
}

CodePudding user response：

begin() returns an iterator by value, not a reference. You are not allowed to form a non-const lvalue reference.

Making it const would prolong the life of the returned iterator and the program would then compile:

const auto &c = str.begin();

On the other hand, iterators are supposed to be cheap to copy and iterators from contiguous containers are often implemented as pure pointers. The idiomatic approach is:

auto c = str.begin();

In your second example, the idiomatic approach to form a reference to the first element would be:

auto& c = str.front();