Trouble understanding static in header files ( inline ) - c++

Question

I've been working with static for quite a long time but now when the header files come I'm a little confused.
The main problem is this code :
#include <iostream>
#include <string>
#include <sstream>
#include <set>
#ifndef WORD_H
#define WORD_H
#include<unordered_map>
class Word
{
private:
std::string word;
int k;
static std::unordered_map<std::string, int> x;
public:
Word(std::string word) : word(word)
{
if (x.find(word) != x.end())
x.insert({ word , 0 });
else
{
x[word]++;
}
}
std::string getWord() const
{
return x.find(this->word)->first;
}
int getCount() const
{
return x.find(this->word)->second;
}
friend bool operator<(const Word& a, const Word& b);
};
bool operator<(const Word& a, const Word& b)
{
return a.getWord() < b.getWord();
}
#endif
std::ostream& operator<<(std::ostream& out, const Word& w) {
return out << w.getWord() << " " << w.getCount();
}
void printWordCounts(const std::string& line) {
std::istringstream lineIn(line);
std::set<Word> words;
std::string wordStr;
while (lineIn >> wordStr) {
words.insert(Word(wordStr));
}
for (Word w : words) {
std::cout << w << std::endl;
}
}
int main() {
std::string line;
std::getline(std::cin, line);
printWordCounts(line);
std::cout << "---" << std::endl;
std::getline(std::cin, line);
printWordCounts(line);
return 0;
}
I know that there is a problem with the static keyword, but what exactly is it?
Also, the error is one of my favourites. Those errors between linker and STL:
Error LNK2001 unresolved external symbol "private: static class std::unordered_map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,int,struct std::hash<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,struct std::equal_to<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,int> > > Word::x" (?x#Word##0V?$unordered_map#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##HU?$hash#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###2#U?$equal_to#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###2#V?$allocator#U?$pair#$$CBV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##H#std###2##std##A) ConsoleApplication63 C:\Users\Ivo\source\repos\ConsoleApplication63\ConsoleApplication63\ConsoleApplication63.obj 1
I hope that someone can give me an explanation.

Answer 1

You have only declared the static member x. You also need to define it outside the class like this:
std::unordered_map<std::string, int> Word::x;
From c++17, you can define x inside the class definition like this:
inline static std::unordered_map<std::string, int> x;

Answer 2

Static variable declarations in classes are identical to declaring a global variable in a header file, except it's 'symbol' lies within the namespace of the class and therefore access to it is protected with visibility levels. This means that like global variable declarations, static class variable declarations have 'weak' linkage and a compilation unit needs to define the symbol definition, so that the linker can tie the memory associated with the variable declarations in other files to exactly one variable.
Defining the static variable is done outside the class like so:
// <variable type> <class type>::<variable name>;
std::unordered_map<std::string, int> Word::x;
Note that only one .cpp file needs to define the variable