Template classes as template parameters
How do I define a template template in container and type?
For example, the overload insert operator for a stream of all elements of a vector, list, or iterator container forwarding:
using namespace std;
#include <iostream>
#include <vector>
#include <list>
//...
//...the second argument is a container template-ed on type T
//...
template <typename T,template <typename U> class C>
ostream&
operator<<
(ostream& p_os,const C<T>& p_c)
{
for(typename C<typename T>::const_iterator cit=p_c.begin();cit!=p_c.end();++cit)
{
p_os.operator<<(*cit);
}
return p_os;
}
int
main
()
{
vector<int> v;
cout << v << endl;
list<int> l;
cout << l << endl;
return 0;
}
This doesn't compile in g ++ 4.9. What's wrong? How it's done?
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Why not just pass the container type as a template parameter and learn the element type from it? In your example code, you don't even need the element type:
template <typename C>
ostream&
operator<<
(ostream& p_os,const C& p_c)
{
typedef typename C::value_type element_type; // if needed
for(typename C::const_iterator cit=p_c.begin();cit!=p_c.end();++cit)
{
p_os.operator<<(*cit);
}
return p_os;
}
(Though it might be unwise to use this for globals like this without some trick enable_if
, as it will match any argument otherwise.)
EDIT: You can, for example, try to restrict this to nested classes value_type
(which all containers have):
template <typename C, typename T = typename C::value_type>
ostream&
operator<<
(ostream& p_os,const C& p_c)
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std::vector
is a class template with two template type parameters:
template <class T, class Alloc = allocator<T> >
class vector;
To make your function work with std::vector
(and other two parameter class templates), you can use the following definition:
template <typename T, typename A, template <typename, typename> class C>
// ~~~~~~~~~^ ~~~~~~~^
ostream& operator<<(ostream& p_os, const C<T,A>& p_c)
// ^^
{
for(typename C<T,A>::const_iterator cit=p_c.begin();cit!=p_c.end();++cit)
{
p_os.operator<<(*cit);
}
return p_os;
}
or alternatively:
template <typename T, template <typename...> class C>
ostream& operator<<(ostream& p_os, const C<T>& p_c);
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Alan Stokes' work is working. The code below can pass any container. I just needed to add an insert operator for cards
using namespace std;
#include <iostream>
#include <vector>
#include <list>
#include <forward_list>
#include <set>
#include <deque>
#include <array>
#include <map>
#include <unordered_map>
//...
//...needed for map types which are (key,value) pairs.
//...
template <typename K,typename V>
ostream&
operator<<
(ostream& p_os,const pair<const K,V>& p_v)
{
std::operator<<(p_os,'(');
p_os << p_v.first;
std::operator<<(p_os,',');
p_os << p_v.second;
std::operator<<(p_os,')');
return p_os;
}
template <typename C, typename T = typename C::iterator>
ostream&
operator<<
(ostream& p_os,const C& p_c)
{
for(typename C::const_iterator cit=p_c.begin();cit!=p_c.end();++cit)
{
typename C::value_type v = *cit;
p_os << v;
std::operator<<(p_os,",");
}
return p_os;
}
int
main
()
{
vector<int> v;
for(int i=0;i<4;++i)
{
v.push_back(i);
}
cout << v << endl;
list<int> l;
for(int i=0;i<4;++i)
{
l.push_back(i);
}
cout << l << endl;
forward_list<int> fl = {0,1,2,3};
cout << fl << endl;
set<int> s;
for(int i=0;i<4;++i)
{
s.insert(i);
}
cout << s << endl;
deque<int> d;
for(int i=0;i<4;++i)
{
d.push_back(i);
}
cout << d << endl;
array<int,4> a = {0,1,2,3};
cout << a << endl;
unordered_map<int,int> um;
for(int i=0;i<4;++i)
{
um[i] = i;
}
cout << um << endl;
map<int,int> m;
for(int i=0;i<4;++i)
{
m[i] = i;
}
cout << m << endl;
return 0;
}
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