如何在容器和类型上定义模板功能?
例如,重载插入运算符以流式传递矢量,列表或转发迭代器容器的所有元素:
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;
}
这不能在g ++ 4.9上编译。怎么了?怎么做?
答案 0 :(得分:1)
std::vector
是一个包含两个模板类型参数的类模板:
template <class T, class Alloc = allocator<T> >
class vector;
要使您的函数与std::vector
(和其他双参数类模板)一起使用,您可以使用以下定义:
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;
}
或者:
template <typename T, template <typename...> class C>
ostream& operator<<(ostream& p_os, const C<T>& p_c);
答案 1 :(得分:1)
为什么不直接将容器类型作为模板参数传递,并从中找出元素类型?在您的示例代码中,您甚至不需要元素类型:
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;
}
(虽然在没有enable_if
技巧的情况下将此用于此类全局函数可能是不明智的,因为它将匹配任何参数。)
编辑:您可以尝试将此限制为具有嵌套value_type
的类(所有容器都有):
template <typename C, typename T = typename C::value_type>
ostream&
operator<<
(ostream& p_os,const C& p_c)
答案 2 :(得分:0)
Alan Stokes的方法很有效。下面的代码可以流式传输任何容器。我只需要为地图添加插入操作符
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;
}