I made a simple program for my study that calculates words in a text file and prints every word and its repeated times in the file.

How can I improve this code?

#include <future>
#include <iostream>
#include <fstream>
#include <unordered_map>
#include <string>
#include <vector>
#include <algorithm>
#include <exception>

typedef std::unordered_map<std::string, std::size_t> Words;

Words wordsInFile(const std::string& fileName)
{
    std::ifstream file(fileName);
    if (!file.is_open())
    {
        throw "Can't open the file" + fileName;
    }

    Words loadFromFile;


    for (std::string word; file >> word;)
    {
        ++loadFromFile[word];
    }

    return loadFromFile;
}

int main(int argc, char *argv[])
{
    std::vector<std::future<Words>> futures;

    for (int i = 1; i < argc; ++i) 
    {
        futures.push_back(std::async([=] { return wordsInFile(argv[i]); }));
    }

    Words words;

    for (auto& i : futures) 
    {
        const auto results = i.get();

        for (const auto& j : results) 
        {
            words[j.first] = j.second;
        }
    }

    std::cout << "Word\tRepeated Times\n-------------------------\n";
    for (const auto& i : words)
        std::cout << i.first << "\t\t" << i.second << '\n';
}

test.txt

File system is huge subject need more work out. file is plain text file
This is some junk words. simple program for words counting in text file. 1 2 3
2 2 2 2
@ % & ^ *
all so good
this test text file

Output:

Word    Repeated Times
-------------------------
File            1
out.            1
1               1
system          1
2               5
is              3
need            1
huge            1
subject         1
work            1
more            1
%               1
file            3
plain           1
text            3
This            1
some            1
junk            1
program         1
words.          1
simple          1
for             1
words           1
counting                1
in              1
file.           1
so              1
3               1
@               1
&               1
^               1
this            1
*               1
all             1
good            1
test            1