Segmentation fault in program which creates a vector from a string

I don't know the reason

You have several issues with your code. The one glaring one is that you failed to return the vector v in the split_words function. Not returning a value from a function that is defined to return a value is undefined behavior.

The second issue is that j falls off the end on the last word, since your loop only stops on s[j] being a blank. The string does not end on a blank character, thus your loop keeps going beyond the length of the string.

Having said this, if your goal is to split a string on the space character, there is no need to write code like this to do the job. Instead, simply use std::istringstream and operator >>:

#include <vector>
#include <sstream>
#include <string>
#include <iostream>

std::vector<std::string> split_words(std::string s) 
{
    std::vector<std::string> v;
    std::istringstream iss(s);
    std::string temp;
    while (iss >> temp)
       v.push_back(temp);
    return v;
}
 
int main() 
{
   std::string s = "Alpha beta! Gamma";
   auto vect = split_words(s);
   for (auto& word : vect)
      std::cout << word << "\n";
   return 0;
} 

Live Example

The loop simply calls operator >> on the stream, and each iteration calls push_back for each parsed string encountered.

I think, provided that you're using C++11 or higher, that you should go for regular expressions, and do something like this:

std::vector<std::string> split_words(std::string s) {
    std::vector<std::string> v;
    std::regex pattern("[!-~]+");
    std::cmatch result;

    while(regex_search(s.c_str(), result, pattern)) {
        for(auto it : result)
           v.push_back(it);
        s = result.suffix().str();
    }

    return v;
}

So that your search matches every single (non-extended) ASCII table character combination except non printable ones, including spaces, and you achieve your aim.

The bug is right here when you get to the last word you forget to check for \0 or the length of the string.

for (int j=i; s[j]!=' ' && j < s.size(); ++j) {
  v[wortanzahl] += s[j];
  i=j;
}

Ah forgot I had seen Alexsandrescu talk about sentinels, so you could actually have solved the problem if you had added a (space) after the input. As first line in your split_words add

s += ' ';

These changes are required. Also mentioned inline in code below.

// 0. can try using namespace std; to clean up code.

// 1. check for end of string in below loop.

// 2. return the vector of strings. (this fixes the crash).

// 3. output the split strings using vector.

#include <string>
#include <vector>
#include <iostream>
#include <algorithm>

// 0. can try using std to clean up code.
std::vector<std::string> split_words(std::string s)
{
    std::vector<std::string> v(1, "");
    int i=0;
    int wortanzahl = 0;
    while(i<s.size())
    {
        if (s[i]!=' ')
        {
            v.resize(wortanzahl+1, "");
            // 1. check for end of string in below loop
            for (int j=i; s[j] && s[j]!=' '; ++j)
            {
                v[wortanzahl] += s[j];
                i=j;
            }
            ++wortanzahl;
        }
        ++i;
    }

    // 2. return the vector of strings
    return v;
}

int main()
{
    std::string s = "Alpha beta! Gamma";
    std::vector<std::string> v = split_words(s);

    // 3. output the split strings using vector
    for (int i = 0; i < v.size(); ++i)
        std::cout << v[i] << std::endl;
    return 0;
}