Passing unspecialized template as a template parameter

Question

Can i do something like

template<class Key, class Data, class Compare = less<Key>, template<typename T> class Allocator<T> = allocator<T> >
    class mymap {
        typedef map<Key,Data,Compare,Allocator<pair<const Key, Data> > > storageMap; 
        typedef vector<Data,Allocator<Data> > storageVector;
}

So template is passed to the class unspecialiazed and instantiated later.

score 4 · Accepted Answer · answered Apr 21 '11 at 14:25

Yes, here's a minimal compileable example:

#include <map>
#include <vector>
using namespace std;

template <
    class Key,
    class Data,
    class Compare = less<Key>,
    template <typename T> class Allocator = allocator
>
class mymap
{
public:
    typedef map<Key,Data,Compare,Allocator<pair<const Key, Data> > > storageMap; 
    typedef vector<Data,Allocator<Data> > storageVector;
};

int main()
{
    mymap<int,long>::storageMap m;
    mymap<int,long>::storageVector v;
    return 0;
}

score 1 · Answer 2 · answered Apr 21 '11 at 14:24

1

Yes, it's called a "template-template parameter", and the syntax is

template <class Key, class Data, class Compare = less<Key>,
          template <typename T> class Allocator = allocator >
class mymap {
    typedef map<Key,Data,Compare,Allocator<pair<const Key, Data> > > storageMap; 
    typedef vector<Data,Allocator<Data> > storageVector;
}

answered Apr 21 '11 at 14:24

Marc Mutz - mmutz

23,707
11
74
87

`code`mymap.hpp(30): error: identifier "T" is undefined template, class Compare = less, template class Allocator = allocator > `code` @mmutz well, gcc isn't happy with such declaration :) – akashihi Apr 21 '11 at 14:30
2

@akashi: `s/= allocator/= allocator/ – Marc Mutz - mmutz Apr 21 '11 at 14:32
my fault, sorry Thanks for assistance – akashihi Apr 21 '11 at 14:39

Passing unspecialized template as a template parameter

2 Answers2

Linked