Template parameters in C++ templates
c++, c++11, template-meta-programming, templates
Solution
Pretty sure this is what you're looking for:
template<template<class, class...> class V, class T, class... Args>
void fn(V<T,Args...>& value)
{
// use value here.
}
Invoked simply as:
std::vector<int> v;
fn(v);
And before you ask, yes, you can do it with templates that need more parameters (like `std::map<>`, etc), Just make sure `Arg...` covers the optionals and you specify the mandatory ones you care about. Such as:
#include <iostream>
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <cstdlib>
template< template<class, class...> class V, class T, class... Args>
void fn_seq(const V<T,Args...>& value)
{
std::cout << __PRETTY_FUNCTION__ << '\n';
// use value here.
}
template< template<class, class, class...> class C, class K, class V, class... Args>
void fn_assoc(const C<K,V,Args...>& value)
{
// use value here.
std::cout << __PRETTY_FUNCTION__ << '\n';
}
int main()
{
std::vector<int> vec;
fn_seq(vec);
std::list<double> lst;
fn_seq(lst);
std::map<int, float> m;
fn_assoc(m);
std::unordered_map<long, long> um;
fn_assoc(um);
return EXIT_SUCCESS;
}
Output
void fn_seq(const V<T, Args...> &) [V = vector, T = int, Args = <std::__1::allocator<int>>]
void fn_seq(const V<T, Args...> &) [V = list, T = double, Args = <std::__1::allocator<double>>]
void fn_assoc(const C<K, V, Args...> &) [C = map, K = int, V = float, Args = <std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, float> >>]
void fn_assoc(const C<K, V, Args...> &) [C = unordered_map, K = long, V = long, Args = <std::__1::hash<long>, std::__1::equal_to<long>, std::__1::allocator<std::__1::pair<const long, long> >>]
Problem
I am trying to use template template parameters, similar to what is done here and here (and many other places). ``` #include <vector> template<template<class> class A, class B> void f(A<B> &value) { } int main() { std::vector<int> value; f<std::vector, int>(value); } ``` But ``` $ g++-4.8 -std=c++0x base64.cpp base64.cpp: In function ‘int main()’: base64.cpp:9:23: error: no matching function for call to ‘f(std::vector<int>&)’ f<std::vector, int>(value); ^ base64.cpp:9:23: note: candidate is: base64.cpp:4:6: note: template<template<class> class H, class S> void f(const H<S>&) void f(H<S> &value) { ``` What am I missing?