Template parameters in C++ templates

问题

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?

回答1:

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> >>]

回答2:

There is another thing you can do instead of modifying the function definition, i.e. use template alias to adapt std::vector:

#include <vector>
#include<iostream>

template< 
  template<class> class A, 
  class B
  >
void f(A<B> &value) {
    for( auto & x : value ) {
      std::cout << x << std::endl;
    }
}

template< class T >
using VectorAdapter = std::vector<T>;

int main() {
    VectorAdapter<int> value {1, 2, 3};
    f(value);
}

回答3:

Vector has 2 template parameters, (the second one is almost never used and has a default allocator type)

template < class T, class Alloc = allocator<T> > class vector;

So it should be:

template<template<class,class> class H, class S,class A>
void f(const H<S,A> &value) {
}

来源：https://stackoverflow.com/questions/21569216/template-parameters-in-c-templates

标签

c++

templates

c++11

template-meta-programming