CODEWITHSUNDEEP
c++constructor
E B
E B

Reputation: 59

Template and map in C++

I have the following code in c++, involving templates : 

 #include <assert.h>
 #include <map>
 #include <limits>

 template<class K, class V> class interval_mapping {
 private:   
    std::map<K,V> aMap;

 public:
    interval_mapping( V const& val) {
        aMap.insert(
            aMap.begin(),
            std::make_pair(std::numeric_limits<K>::min(),val)
        );
    }

  void assign( K const& keyBegin, K const& keyEnd, const V& val ) {}

   V const& operator[]( K const& key ) const {
         return ( --m_map.upper_bound(key) )->second;
   }
 };

int main()
{
     interval_mapping<int, char> myMap;
}

I get the error:

no default constructor exists for class "interval_mapping"

Upvotes: 1

Views: 834

Answers (3)

Ross Smith
Ross Smith

Reputation: 3765

The error should be pretty self-explanatory: you're constructing an interval_mapping object without passing any arguments, so the class needs a default constructor. Default constructors are only auto-generated by the compiler if you don't provide any other constructors yourself.

You don't need the default constructor to do anything special, so you can just add

interval_mapping() {}

to the class, or

interval_mapping() = default;

if your compiler supports C++11.

If you genuinely didn't want the class to have a default constructor, you need to give an argument in the line in main() where you create the object.

Upvotes: 2

YoungJohn
YoungJohn

Reputation: 956

You declare a constructor for interval_mapping that takes a V const& val. This is the only constructor declared for interval_mapping and no default constructors are provided since you have implemented your own.

Then when you try to instantiate an interval_mapping<int, char> you fail to provide the V const& val needed by the constructor (in this case that would be a char it seems). And since there is no available constructor that takes no parameters you see this error.

Upvotes: 0

Mike Seymour
Mike Seymour

Reputation: 254431

As the error says, there's no default constructor, since you've declared your own constructor.

Either use that constructor

interval_mapping<int, char> myMap(some_char_value);

or add a default constructor; you probably want it to be empty initially:

interval_mapping() {}

Upvotes: 2

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