CODEWITHSUNDEEP
c++
Tom
Tom

Reputation: 393

confusion about how to use c++ reference

I am new to C++, I got confused about the c++ reference, for example, the std::map::insert reference, at the beginning, it has :

pair<iterator,bool> insert (const value_type& val);
template <class P> pair<iterator,bool> insert (P&& val);    
iterator insert (const_iterator position, const value_type& val);
template <class P> iterator insert (const_iterator position, P&& val);
template <class InputIterator>
void insert (InputIterator first, InputIterator last);  
void insert (initializer_list<value_type> il);

In the later example, it uses the insert like this:

mymap.insert ( std::pair<char,int>('z',200) );

From which line of the reference can I know that I can use the insert function as above?

Upvotes: 1

Views: 212

Answers (3)

Columbo
Columbo

Reputation: 60969

std::pair<char const, int> is the value_type of mymap. The first insert overload you quoted takes a const-reference to value_type.

Using a standard library reference requires basic knowledge of the language and corresponding library parts. Looking up a member function without understanding of the class it is a member of will, in most of the cases, not work out well. If you have a look at the reference for map you'll see that value_type is defined as pair<const key_type, mapped_type> which fits here since pair<char, int> is convertible to pair<char const, int> -- you also need to know how pair works.

Upvotes: 1

Marco A.
Marco A.

Reputation: 43662

Assuming that you have a std::map<char,int>, the line

mymap.insert ( std::pair<char,int>('z',200) );

is allowed since map defines the single element constructor

single element (1)  
pair<iterator,bool> insert (const value_type& val);
template <class P> pair<iterator,bool> insert (P&& val);

the second one should kick in if you're using C++11. In C++03 only the first one is available.

The documentation says

Member type value_type is the type of the elements in the container, defined in map as 
pair<const key_type,mapped_type>

thus std::pair<char,int> is the value_type of your map and the insertion is valid (a temporary can bind to a lvalue const reference or rvalue references in C++11).

I'm not suggesting to also take a look at cppreference since it might be even less understandable if you had problems with cplusplus.com, but generally I would recommend it.

Upvotes: 1

CinCout
CinCout

Reputation: 9619

If you read the Parameters section on the same page below, you will find the following description for val:

Value to be copied to (or moved as) the inserted element. Member type value_type is the type of the elements in the container, defined in map as pair<const key_type,mapped_type>

which clearly states that value_type is the type of the elements in the container, defined in map as pair<const key_type,mapped_type>

Upvotes: 1

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