[HH] c++ strings?

[Matthew Gillen]

No, you have to mark your function as not modifying the argument:
  void Note(const string &msg) { ...

Most functions I've written in C++ have had const parameters.

Now, if you want the function to modify the parameter, but only a local 
copy of the parameter, then you should pass by value (i.e. omit the '&' 
and the 'const')

Matt

On 11/20/2012 04:15 PM, Greg London wrote:
>
> So, if I have a subroutine that passes by reference,
> and I want to pass in something that can't be modified,
> I have to put it into some intermediate variable first?
>
> That seems.... inefficient.
>
>
>
>> 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
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>
>