CODEWITHSUNDEEP
c++templatesaliastypedeftypename
dseiple
dseiple

Reputation: 638

C++ Type alias error: expected unqualified-id before 'using'

There are numerous resources explaining how to do Type aliasing, but clearly I am missing something. I am trying to create a map alias. When debugging I want my Map_t to be the standard library map since it is easier to see what it contains, and when not debugging I want it to be btree::btree_map since it uses less memory.

To do this I tried the following:

#if DEBUG
#include <map>
template<typename U, typename V>
using Map_t = map<typename U, typename V>;  // <--- Error on this line
#else
#include "btree_map.h"
template<typename U, typename V>
using Map_t = btree::btree_map<typename U,typename V>;  // <--- Error on this line
#endif

When compiling I get the error error: expected unqualified-id before 'using'. I've tried several variations of the above with no success. Thanks in advance for your comments.

UPDATE: I feel I may have made this a little confusing be including the btree portion. SO let me expand a little. If I declare my types as say either

btree::btree_map<int, int> mymap;

or

map<int, int> mymap;

all is fine. But just trying to introduce

template<typename U, typename V>
using Map_t = map<U, V>;

Map_t<int, int> mymap;

leads the the error.

Upvotes: 1

Views: 3991

Answers (1)

juanchopanza
juanchopanza

Reputation: 227400

Your syntax is slightly wrong. You need

template<typename U, typename V>
using Map_t = map<U, V>;

As for the btree segment, it depends on the details of btree. If it is a namespace with a class template called btree_map, then you need

template<typename U, typename V>
using Map_t = btree::btree_map<U, V>;

If it is a class template with an inner type, then you may need something like

template<typename U, typename V>
using Map_t = typename btree<U,V>::btree_map;

Upvotes: 1

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