What does iterator- second mean

Question

In C    what is the type of a std  map lt  gt   iterator   We know that an object it of type std  map lt A B gt   iterator has an overloaded operator - gt  which returns a std  pair lt A B gt    and that the std  pair lt  gt  has a first and second member   But  what do these two members correspond to  and why do we have to access the value stored in the map as it- gt second

User · Answer

The type of the elements of an std  map  which is also the type of an expression obtained by dereferencing an iterator of that map  whose key is K and value is V is std  pair lt const K  V gt  - the key is const to prevent you from interfering with the internal sorting of map values   std  pair lt  gt  has two members named first and second  see here   with quite an intuitive meaning  Thus  given an iterator i to a certain map  the expression   i- gt first   Which is equivalent to     i  first   Refers to the first  const  element of the pair object pointed to by the iterator - i e  it refers to a key in the map  Instead  the expression   i- gt second   Which is equivalent to     i  second   Refers to the second element of the pair - i e  to the corresponding value in the map

User · Answer

I m sure you know that a std  vector lt X gt  stores a whole bunch of X objects  right  But if you have a std  map lt X  Y gt   what it actually stores is a whole bunch of std  pair lt const X  Y gt s  That s exactly what a map is - it pairs together the keys and the associated values   When you iterate over a std  map  you re iterating over all of these std  pairs  When you dereference one of these iterators  you get a std  pair containing the key and its associated value   std  map lt std  string  int gt  m      fill it     auto it   m begin      Here  if you now do  it  you will get the the std  pair for the first element in the map   Now the type std  pair gives you access to its elements through two members  first and second  So if you have a std  pair lt X  Y gt  called p  p first is an X object and p second is a Y object   So now you know that dereferencing a std  map iterator gives you a std  pair  you can then access its elements with first and second  For example    it  first will give you the key and   it  second will give you the value  These are equivalent to it- gt first and it- gt second

[c++] What does iterator->second mean?

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