[HH] c++ strings?

[Matthew Gillen]

On 11/20/2012 03:57 PM, Matthew Gillen wrote:
> On 11/20/2012 03:41 PM, Greg London wrote:
>> Why would I need "const" when I pass by reference, but not need const
>> when
>> I pass by value?
>
> Because references are not pointers, and you cannot "re-seat" the
> reference.
>    http://www.parashift.com/c++-faq/reseating-refs.html
>
> If you want the nitty-gritty details on how references are implemented:
>   http://www.parashift.com/c++-faq/overview-refs.html

Sorry, I jumped the gun.  Your problem had nothing to do with re-seating 
a reference.

The problem in your example:
  ...
  Note("hello world\n");
  ...
  void Note( string &msg){ ...

is that "hello world", by virtue of being a string literal, *is* a const 
string, but your function prototype (when sans-const) is indicating that 
the function reserves the right to modify the string.

When you pass by value, you're always getting a copy, so the effect is 
essentially:
  Note(new std::string("hello world\n");

...and there's never a problem modifying your local copy of the original 
string. However, if you pass by reference, you're not making a new copy, 
so if the source object has some restrictions (e.g. is const), then you 
can't pass it to functions that reserve the right to modify it.

Const-correctness can be tricky when you're first learning it, but if 
you get it right it is a great tool to make the the compiler help you.

Matt