CODEWITHSUNDEEP
c++templates
Niranjan Kotha
Niranjan Kotha

Reputation: 309

Can we declare a class template using fewer parameters than in the template?

Class template for map is like this

template < class Key,                                     // map::key_type
       class T,                                       // map::mapped_type
       class Compare = less<Key>,                     // map::key_compare
       class Alloc = allocator<pair<const Key,T> >    // map::allocator_type
       > class map;

From the template it appears to take 4 template parameters but while declaring a map, I see only two template parameters. 

map <int, int> table;

Is this not a problem? If its not is this correct to just define the type of last two(of the four)?

map<,,int,int>

Upvotes: 2

Views: 247

Answers (3)

songyuanyao
songyuanyao

Reputation: 172924

  1. Is this not a problem?

This is not a problem, it's default template arguments.

Default template arguments are specified in the parameter lists after the = sign.

It means when you doesn't specify them, such as map<key_type, value_type>, the default arguments will be used instead. i.e. Compare will be less<key_type>, and Alloc will be allocator<pair<const key_type, value_type>>.

  1. is this correct to just define the type of last two(of the four)? map<,,int,int>

No. Default arguments are used in place of the missing trailing arguments. You can't just specify the last two arguments, without specifying the first two and want the default arguments to be applied. And in this case the first two parameters don't have default argument at all.

BTW

If the default is specified for a template parameter of a primary class template, each subsequent template parameter must have a default argument, except the very last one may be a template parameter pack.

Upvotes: 7

Joe
Joe

Reputation: 410

This is the default parameter for the template.

But only one or more in the end could be the default parameter(i.e. once a default parameter exists, all of the parameters after this parameter must be the default parameter.)

so, your code like this :

map<,,int,int>

will be wrong.

the right code like this:

map<int,int>;

note: you can refer to Thinking.In.C++,.Second.Edition.Volume.2

Upvotes: 2

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

Is this not a problem?

No, that's what the default type definitions are for: = less<Key> and = allocator<pair<const Key,T> >.

As you can see these types can be deduced from Key and T.

If its not is this correct to just define the type of last two(of the four)? map<,,int,int>

No that can't be done.

Upvotes: 5

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