On Sat, 29 Jan 2005, Shachar Shemesh wrote:
> Hi all,
>
> Here is a small program for your viewing pleasure:
>
> > class a {
> > public:
> > explicit a(int param);
What is the meaning of 'explicit' declaration?
Is this a C++ keyword which was added since I learned C++?
> >
> > a &operator= ( a &that );
> > };
How are the variables in this class declared (if there are any variables
at all)?
> >
> > int main()
> > {
> > a var1(3);
> >
> > var1=a(5);
> >
> > return 0;
> > }
>
> Somewhat surprisingly, this does not compile:
> g++ -Wall -g testcompile.cc -o testcompile
> testcompile.cc: In function `int main()':
> testcompile.cc:12: error: no match for 'operator=' in 'var1 = a(5)'
> testcompile.cc:5: error: candidates are: a& a::operator=(a&)
> make: *** [testcompile] Error 1
>
> There are two things that can make it compile. One is to add a "const"
> at the "operator=" definition, and the other is to use an explicit
> variable (i.e. - not a temporary one).
>
> The reason for this failure seems to be that g++ treats temporary
> variables as consts. I see neither reason nor logic for this decision,
> however. Why can't I modify temporary variables if I so wish? Don't they
> have a well defined life span (until the end of the statement) for a reason?
My guess is that the language allows the temporary a(5) to be compiled as
a constant and stored in read-only part of the program.
Consider what you would have wished to happen if you had used
complex(0.707,0.707) instead of your own a.
--- Omer
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