I have a function in my namespace ns
that helps me print STL containers. For example:
template <typename T>
std::ostream& operator<<(std::ostream& stream, const std::set<T>& set)
{
stream << "{";
bool first = true;
for (const T& item : set)
{
if (!first)
stream << ", ";
else
first = false;
stream << item;
}
stream << "}";
return stream;
}
This works great for printing with operator <<
directly:
std::set<std::string> x = { "1", "2", "3", "4" };
std::cout << x << std::endl;
However, using boost::format
is impossible:
std::set<std::string> x = { "1", "2", "3", "4" };
boost::format("%1%") % x;
The problem is fairly obvious: Boost has no idea that I would like it to use my custom operator <<
to print types which have nothing to do with my namespace. Outside of adding a using
declaration into boost/format/feed_args.hpp
, is there a convenient way to make boost::format
look for my operator <<
?
The solution I actually went with is quite similar to Answeror's, but it works for anything:
namespace ns
{
template <typename T>
class FormatWrapper
{
public:
explicit FormatWrapper(const T& x) :
ref(x)
{ }
friend std::ostream& operator<<(std::ostream& stream,
const FormatWrapper<T>& self
)
{
// The key is that operator<< is name lookup occurs inside of `ns`:
return stream << self.ref;
}
private:
const T& ref;
};
template <typename T>
FormatWrapper<T> Formatable(const T& x)
{
return FormatWrapper<T>(x);
}
}
So usage is:
boost::format("%1%") % Formatable(x);
I think the most clean way is to provide a thin wrapper in your own namespace for each of the operators you want to override. For your case, it can be:
namespace ns
{
namespace wrappers
{
template<class T>
struct out
{
const std::set<T> &set;
out(const std::set<T> &set) : set(set) {}
friend std::ostream& operator<<(std::ostream& stream, const out &o)
{
stream << "{";
bool first = true;
for (const T& item : o.set)
{
if (!first)
stream << ", ";
else
first = false;
stream << item;
}
stream << "}";
return stream;
}
};
}
template<class T>
wrappers::out<T> out(const std::set<T> &set)
{
return wrappers::out<T>(set);
}
}
Then use it like this:
std::cout << boost::format("%1%") % ns::out(x);
You can try something like this:
namespace boost // or __gnu_cxx
{
using np::operator<<;
}
#include <boost/format/feed_args.hpp>
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