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From: Greg Chicares <gchicares@sb...> - 2007-11-26 04:45:47
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On 2007-11-25 17:36Z, John Brown wrote:
[...]
> /* 09 */ template <class T>
> /* 10 */ T range_sum(T a[], int beg, int end, T initial=T())
> /* 11 */ {
> /* 12 */ T sum = 0;
> /* 13 */ for (int i=beg; i<=end; i++)
> /* 14 */ sum += a[i];
> /* 15 */ return sum + initial;
> /* 16 */ }
> /* 17 */
> /* 18 */ // explicit instantiation
What follows is an explicit specialization, not an explicit
instantiation. These would be explicit instantiations:
template int range_sum(int a[], int beg, int end, int initial);
template double range_sum(double a[], int beg, int end, double initial);
> /* 19 */ template <>
> /* 20 */ int range_sum(int a[], int beg, int end, int initial=int())
> /* 21 */ {
> /* 22 */ int sum = 0;
> /* 23 */ for (int i=beg; i<=end; i++)
> /* 24 */ sum += a[i];
> /* 25 */ return sum + initial;
> /* 26 */ };
Stray semicolon on line 26, BTW.
> When I compile this, I get:
> 06-RangeSum.cpp:20: error: default argument specified in explicit specialization
I believe the operative rule is 14.7.3/21:
"Default function arguments shall not be specified in [...]
the explicit specialization of a function template"
> My questions:
>
> 1) I understand that when you write an explicit specialisation, you are supposed
> to write 'template<> ...' and not, for example 'template<int> ..., but I
> tried it. That is, I changed lines 19 and 29 to 'template' and 'template
> respectively. The error at line 20 went away, but the error at line 30
> remained. The error also goes away if I set T = char. However, double and
> float give the 'default argument' error. Why is this?
I'm guessing that "hotmail" altered that text, and that you
actually changed those lines to look like this:
/* 19 */ template<int>
/* 29 */ template<double>
Those would define new and different function templates, with
non-type arguments. The first is syntactically valid, but it
almost certainly doesn't do what you want. The second isn't
valid because a non-type argument can't be a double [14.3.2/1],
and would have given a different error message, e.g.
`double' is not a valid type for a template constant parameter
at least if you got rid of the default argument.
> 2) If I do not provide any explicit specialisations, the program will
> compile and produce the expected output:
>
> 12
> 12.00
>
> So why does it complain when I provide them?
Just because the default arguments were respecified.
But what are you trying to accomplish here?
Why define partial specializations that just repeat the generic
code, plugging in a particular type? The point of C++'s template
facility is that the compiler does that for you.
And why not just use std::accumulate() instead of writing this
function template at all?
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