Re: cout << vector<string>
On Nov 7, 7:21 pm, Jeff Schwab <j...@schwabcenter.com> wrote:
James Kanze wrote:
On Nov 7, 1:32 pm, Jeff Schwab <j...@schwabcenter.com> wrote:
Pete Becker wrote:
On 2008-11-07 06:03:15 -0500, Maxim Yegorushkin
<maxim.yegorush...@gmail.com> said:
The example probably assumes there is an overloaded operator<<() for
std::ostream and std::vector<>, something like this:
namespace std {
template<class A1, class A2>
ostream& operator<<(ostream& s, vector<A1, A2> const& vec)
Which has undefined behavior. You can only add template
specializations to namespace std when they depend on
user-defined types.
The correct alternative, AIUI:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
template<typename T>
std::ostream& operator<<(std::ostream& out, std::vector<T> const& v) {
if (!v.empty()) {
typedef std::ostream_iterator<T> out_iter;
copy(v.begin(), v.end() - 1, out_iter( out, " " ));
out << v.back();
}
return out;
}
int main() {
int const ints[] = { 1, 2, 3, 4 };
std::cout << std::vector<int>( ints, ints + 4 ) << '\n';
}
Correct in what sense? It's OK for simple test, like this,
but it's certainly not something you'd allow in production
code.
Correct in the sense that it implements the OP's intent, and
is allowed by the standard. As far as using it in production
code... Ad hoc output of this sort only really comes up for
debugging purposes. If you don't even think it's OK for
debug, I'd love to know your suggested alternative.
Do you leave such debugging code in your production code? If
not, no problem, but there's no point in making the << a
template. If so:
-- It doesn't work. Try it with std::vector<
std::vector< int > >, for example.
-- It very rapidly introduces undefined behavior, as you write
the above, and some other programmer provides the same
function, but with a comma as a separator.
For a quick debugging session, the simplest solution is to knock
out a non-template version in an unnamed namespace. For
anything else, you really need to define what is wanted; you
don't output raw vectors, but vectors with a semantic
signification, which determines how you format them. If you're
using vectors to represent both sets and sequences in a
mathematical context, for example, you'll doubtlessly have a
class for each, with the vector buried deep down in it. And
those classes will provide the formatted output.
If you find that you often need such debugging output (I don't,
but I'm sure that there are applications that to), then you
might want to come up with a general FormattedSequence class
template, with a template function to generate it (so you get
automatic type deduction); this could, of course, be in any
namespace you want.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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