Is it legal to write to std::string?

In std::string there are only const members to fetch the data like c_str(). However I can get a reference to the first element of the string via operator[] and I can write to it.

For example, if I have function:

void toupper(char *first,char *last_plus_one);

I can write directly to vector getting a pointer to the first element:

vector<char> message // has "Some Message";
toupper(&message[0],&message[0]+message.size());

Can I do same thing with std::string?

string message="Some Message";
toupper(&message[0],&message[0]+message.size());

Does the standard guarantee that the location of the memory is actually linear? ie:

&(*(message.begin()+n)) == &message[n]

Thanks.