How to connect a boost signal for only once? - boost

Question

How do I achieve the Qt::UniqueConnection mechanism in Boost?
Basically, I only want to connect the signal for only once.

Answer 1

As far as I know, there's no such thing, but you can easily mimic it using a custom combiner.
1. Using the custom Combiner
You can just ignore any excess slots. This combiner, for example, just ignores anything beyond the first slot (that is, the first added with at_front and no group, or the first in the group with the highest priority, see Ordering Slot Call Groups):
template<typename Result>
struct call_first {
typedef boost::optional<Result> result_type;
template<typename It>
result_type operator()(It first, It last) const {
if (first != last)
return *first;
return boost::none;
}
};
Now here's a test case: Live On Coliru
#include <iostream>
#include <boost/optional/optional_io.hpp>
int foo(int i) { std::cout << "(foo)"; return i*2; }
int bar(int i) { std::cout << "(bar)"; return i+2; }
int main()
{
using namespace boost::signals2;
signal<int(int), call_first<int> > sig;
std::cout << sig(42) << "\n"; // should be none ("--")
sig.connect(foo);
std::cout << sig(42) << "\n"; // should return 42*2
sig.connect(bar);
std::cout << sig(42) << "\n"; // should still return 42*2
}
Printing
--
(foo) 84
(foo) 84
2. Fix your connection management
You can make it easy to avoid double-connecting a signal, by using pretty simple helpers (assuming c++11 for this sample):
template <typename Sig, typename F> void try_connect(Sig& sig, F&& f) {
if (!sig.num_slots()) sig.connect(std::forward<F>(f));
}
template <typename Sig, typename F> void re_connect(Sig& sig, F&& f) {
sig.disconnect_all_slots();
sig.connect(std::forward<F>(f));
}
The corresponding test case Live On Coliru:
int main()
{
using namespace boost::signals2;
signal<int(int)> sig;
std::cout << sig(42) << "\n"; // should be none ("--")
try_connect(sig, foo);
std::cout << sig(42) << "\n"; // should return 42*2
try_connect(sig, bar);
std::cout << sig(42) << "\n"; // should still return 42*2
re_connect(sig, bar);
std::cout << sig(42) << "\n"; // should return 42+2
}
Output:
--
(foo) 84
(foo) 84
(bar) 44