CODEWITHSUNDEEP
c++vectorunordered-map
Lama.ahc
Lama.ahc

Reputation: 33

Vector to a string and HashMap in C++

I am beginning with vector and unordered_map in C++. But I don't understand the reason of the error when accessing a value of the map by its key.

First I define a vector of integers as follows :

std::vector <int> vec;
vec.push_back(1);
vec.push_back(5);
vec.push_back(1);

then I convert the vector to a string: 

std::ostringstream oss;
if (!vec.empty())
   std::copy(vec.begin(), vec.end(),std::ostream_iterator<int>(oss));

std::string s = oss.str();
const void * s1=s.c_str();

and I create my map : 

std::unordered_map<const void *, const void *> map1;

map1[s1]="Hello";

Here, if s1 is equal to "151", why can't I obtain the value "Hello" of the key "151" like this :

std::cout << (char *) map1["151"] << std::endl;

unlike this :

const char* s2 = "180"
map1[s2]="World"
std::cout <<"The value of the key '180' :" << (char *) map1["180"] << std::endl;

Here, I can have "The value of the key '180' : World" displayed.

So what is really s1 ? Isn't it the string "151" ?

Upvotes: 2

Views: 1113

Answers (1)

songyuanyao
songyuanyao

Reputation: 172894

From the view of a std::unordered_map<const void *, const void *>, map1[s1] is different from map1["151"], because the unordered_map just compare the key by the value of the pointer, not the value the pointer points to.

And in the 2nd code, s2 points to the literal c-style string "180", so map1[s2] and map1["180"] could get the same value, because they're pointing to the same thing.

According to the declaration of std::unordered_map,

template<
    class Key,
    class T,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator< std::pair<const Key, T> >
> class unordered_map;

std::unordered_map use std::equal_to as its default key comparer, which just use operator== for comparing.

You can provide your own comparer for std::unordered_map to change the behaviour, like

std::unordered_map<const void*, const void*, std::hash<const void*>, my_comparer> map1;

Upvotes: 4

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