torhu wrote:
In 'The C++ Programming Language', Special Edition, there is an
example in chapter 11.8, page 286 that is meant to illustrate the
subscript operator (operator[]). It is a class that associates
strings with doubles. It looks like this:
class Assoc {
public:
const double& operator[](const string&);
double& operator[](string&);
//and some other stuff...
};
There are nother other operator[]'s declared. I have two questions:
1. Why does one function return a const double& when the function
itself is not declared const? What purpose does this serve?
Returning a "const double&" prevents the client from modifying the
internal value. This has nothing to do with a function being const.
A function declared as "const" promises not to modify any of the
class' data members.
2. What is the point of using what looks to me as a non-const
input-only argument in the second function?
Why not like this:
double& operator[](const string&);
and maybe this too ?:
const double& operator[](const string&) const;
Is it just me being slow? I have pondered this problem for weeks
now...
:)
The second function returns a reference to a value. Since it is
returning a reference, the value inside the function can be
modified by the client using the function.
Try this:
#include <iostream>
using std::cout;
using std::ostream;
class Example_Class
{
double my_var;
public:
Example_Class(double new_value)
: my_var(new_value)
{ ; }
double & paradox(void)
{return my_var;}
friend ostream& operator<<(ostream& out,
const Example_Class& ec);
};
ostream& operator<<(ostream& out,
const Example_Class& ec)
{
out << ec.my_var;
return out;
}
int main(void)
{
Example_Class example(3.14159);
cout << "Original value: " << example << "\n";
example.paradox() = 1.1414;
cout << "After paradox(): " << example << "\n";
return 0;
}
The key issue here is "references". The method is not
returning a _copy_ of the item, but a _reference_ to
the item. Although references can be more convenient
than passing copies, they do have their suprises as
show above. To allow the convenience of not making
a copy of the value and prevent the client from changing
the value, the return type is a const reference.
--
Thomas Matthews
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