C ++: using type as map key introduces template replacement errors
I use a tuple as a key to keep track of items in the map, and then iterate over the map to create a string version of the map. To help me with the transformation, I have a template convenience function that concatenates a tuple used as a key (inspired by this answer ).
#include <iostream>
#include <string>
#include <sstream>
#include <tuple>
template<std::size_t idx = 0, typename ...T>
inline typename std::enable_if<idx == sizeof...(T), void>::type
cat_tuple(std::tuple<T...> &t, std::stringstream &s){
}
template<std::size_t idx = 0, typename ...T>
inline typename std::enable_if<idx < sizeof...(T), void>::type
cat_tuple(std::tuple<T...> &t, std::stringstream &s){
if (idx != 0)
s << ",";
s << std::get<idx>(t);
cat_tuple<idx+1, T...>(t, s);
}
typedef std::tuple<int, int> my_tuple;
int main(){
my_tuple t(1, 2);
std::stringstream s;
cat_tuple(t, s);
std::cout << s.str() << std::endl; //Correctly prints "1,2"
}
I can insert items into a map and iterate over without error
#include <iostream>
#include <string>
#include <sstream>
#include <tuple>
#include <map>
typedef std::tuple<int, int> my_tuple;
typedef std::map<my_tuple, int> my_map;
int main(){
my_map m;
my_tuple t(1, 2);
m.insert(std::pair<my_tuple, int>(t, 1));
std::stringstream s;
for(my_map::iterator i = m.begin(); i != m.end(); ++i)
s << i->second;
std::cout << s.str() << std::endl; //Correctly prints "1"
}
But when I try to iterate over the map, I get a replacement error:
#include <iostream>
#include <string>
#include <sstream>
#include <tuple>
#include <map>
template<std::size_t idx = 0, typename ...T>
inline typename std::enable_if<idx == sizeof...(T), void>::type
cat_tuple(std::tuple<T...> &t, std::stringstream &s){
}
template<std::size_t idx = 0, typename ...T>
inline typename std::enable_if<idx < sizeof...(T), void>::type
cat_tuple(std::tuple<T...> &t, std::stringstream &s){
if (idx != 0)
s << ",";
s << std::get<idx>(t);
cat_tuple<idx+1, T...>(t, s);
}
typedef std::tuple<int, int> my_tuple;
typedef std::map<my_tuple, int> my_map;
int main(){
my_map m;
my_tuple t(1, 2);
m.insert(std::pair<my_tuple, int>(t, 1));
std::stringstream s;
for(my_map::iterator i = m.begin(); i != m.end(); ++i){
if (i != m.begin())
s << "\n";
cat_tuple(i->first, s); //Substitution error, see below
s << " " << i->second;
}
std::cout << s.str() << std::endl;
}
(g ++ 4.2.1 on OSX, removed extraneous notes enable_if)
$ g++ asdf.cc -std=c++11
asdf.cc:31:3: error: no matching function for call to 'cat_tuple'
cat_tuple(i->first, s); //Substitution error, see below
^~~~~~~~~
...
asdf.cc:14:1: note: candidate template ignored: substitution failure [with idx =
0, T = <int, int>]
cat_tuple(std::tuple<T...> &t, std::stringstream &s){
^
1 error generated.
As the error message says, the template I want to use has been ignored due to a replacement failure. What difference did I imagine when I moved it to the map, and how would I fix it?
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Change both function templates cat_tuple
to
cat_tuple(std::tuple<T...> const& t, std::stringstream &s)
// ^^^^^
This is necessary because the keys are std::map
const
.
The parameter cat_tuple
tuple
should be there anyway const&
, since it doesn't change the argument.
You can rename cat_tuple
to tuple_printer
or something like that and change the parameter std::stringstream
to std::ostream
so you can pass it std::cout
to the output if you like stdout
.
Also, the current name is too similar std::tuple_cat
.
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