MESSAGE
| DATE | 2015-09-07 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [LIU Comp Sci] C++ Fwd: Re: References to const
|
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Date: Mon, 07 Sep 2015 13:48:27 -0400
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Subject: [LIU Comp Sci] C++ Fwd: Re: References to const
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-------- Forwarded Message --------
Subject: Re: References to const
Date: Wed, 29 Jul 2015 20:41:48 CST
From: Richard
Reply-To: Richard
Organization: multi-cellular, biological
Newsgroups: comp.lang.c++.moderated
References:
[Please do not mail me a copy of your followup]
Robert spake the secret code
thusly:
>C++ Primer page 61 "..we can initialize a reference to const from any
>expression that can be converted to the type of the reference. In
>particular, we can bind a reference to const to a nonconst object, a
>literal, or a more general expression."
>
>What is the reason for this exception?
It means that you can have a function or method that accepts a parameter by
const reference and call that function or method with an instance of a
non-const object.
This is really the only way that it makes sense to use const references;
otherwise the only thing you could associate with a const reference would be
a const object and most objects change at *some* point in the program.
Consider the following function:
bool ends_with_foo(const std::string &text) {
return text.size() >= 3U && text.substr(text.size()-3) == "foo"; }
This function is saying that it's parameter 'text' is obtained by a const
reference to std::string. The const means we promise not to modify the
argument inside the function -- which is good because this function is
acting like a predicate and it shouldn't be changing the object under
inspection.
If we could only call 'ends_with_foo' on const strings, this would be
unfortunate, because we'd end up doing needless copies just to be able to
call this predicate. The rule that allows a const reference to be
initialized from a non-const object makes the predicate useful in the
obvious way:
std::string ender{"I end with foo"};
std::string unender{"I end with bar"};
bool yes = ends_with_foo(ender);
bool no = ends_with_foo(unender);
Otherwise I'd have to get things into a constant string just to be able to
query it:
std::string ender{"I end with foo"};
const std::string const_ender{ender};
std::string unender{"I end with bar"};
const std::string const_unender{unender}; bool yes =
ends_with_foo(const_ender); bool no = ends_with_foo(const_unender);
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