[HH] c++ strings?

[Jerry Feldman]

Totally agree.
One other thing is that C++ could also create a few copies under the
covers.
For instance:
    Note("Hello, World!")
    A C string literal is passed in, then a std::string is created.
I can't think of a good example to show this at the moment.
It's time tp go home


On 11/20/2012 04:21 PM, Matthew Gillen wrote:
> 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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>>>
>>
>>
>
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-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
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