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I want to define a function that accepts different types of containers: those that own the data (like std::vector) and those that don’t (like boost::iterator_range).
I wrote the following:
#include <vector>
#include <boost/range/iterator_range.hpp>
template<typename C>
void fill(C& c, typename C::value_type v){
for (auto& j : c)
j = v;
}
template<typename C> // How to avoid this implementation?
void fill(const C& c, typename C::value_type v){
for (auto& j : c)
j = v;
}
int main(){
std::vector<int> v(10);
auto rng = boost::make_iterator_range(v.begin(), v.end());
fill(v, 1); // Case 1 -- Calling on std::vector
fill(rng, 2); // Case 2 -- Calling on boost::iterator_range
// Case 3 -- Calling with an r-value, requires the overload
fill(boost::make_iterator_range(v.begin(), v.end()), 3);
// Case 4 -- Should not be allowed
// fill(std::vector<int>(10), 4);
}
My first implementation of fill works well, in that it accepts both kinds of iterators.
However, since boost::iterator_range does not own the data, it can be const (while the data is non-const) and so I would like to allow rvalues as arguments. This means that I need the second implementation.
Is this the only way to do this? Is there a way to avoid the second implementation?
Answer
You can use perfect forwarding:
template <typename C>
void fill(C&& c, typename std::decay_t<C>::value_type v) {
for (auto& j : c)
j = v;
}
This isn’t actually “perfectly forwarding” since you’re not looking to take advantage of move semantics, if you wanted to you should write
std::forward<C>(c)instead ofc¹
#include <boost/range/iterator_range.hpp>
#include <vector>
template <typename C>
void fill(C&& c, typename std::decay_t<C>::value_type v) {
for (auto& j : c)
j = v;
}
int main() {
std::vector<int> v(10);
auto rng = boost::make_iterator_range(v.begin(), v.end());
fill(v, 1); // Case 1 -- Calling on std::vector
fill(rng, 2); // Case 2 -- Calling on boost::iterator_range
// Case 3 -- Calling with an l-value, requires the overload
fill(boost::make_iterator_range(v.begin(), v.end()), 3);
}
¹ Note that you have to be careful not to use-after-move if you do.