Exited with return code issue

[programmer12002]

I'm very new to coding and am trying to do this assignment where all the non alpha characters must be removed from the input string. So if the input is:

-Hello, 1 world$!

The output would be:

Helloworld

I have the following written:

#include <iostream>
#include <string>
#include <cctype>
using namespace std;

int main() {
   
   string str;
   int strSize;
   bool a;
   
   getline(cin,str);
   string finalStr = str;
   strSize = str.size();
   
   for (int i = 0; i < strSize; i++) {
      a = isalpha(str.at(i));
      if (a == false) {
         finalStr.erase(i,1);
      }
   }
   cout << finalStr;

   return 0;
} 

But an error occurs and I am not sure what is causing the error nor how to fix it. This is the error message:

Exited with return code -6 (SIGABRT).
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::erase: __pos (which is 15) > this->size() (which is 12)

Again I'm very new to this so I really haven't an idea of where to even start other than assuming its an overflow issue. Also I got no idea how the formatting on the code works sorry.

[Ganado]

You will receive faster, better help if you format your source code to make it easier to read.
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[AbstractionAnon]

finalStr.erase(i,1); reduces the size of the string.
Therefore, you're going to index out of bounds because you don't adjust the upper limit of your loop.

[seeplus]

Making the new string can be done using one std::algorithm function.

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
#include <iterator>

int main() {
    std::string str, finalStr;

    std::getline(std::cin, str);
    std::remove_copy_if(str.begin(), str.end(), std::back_inserter(finalStr), [](unsigned ch) {return !std::isalpha(ch); });
    std::cout << finalStr << '\n';
}

-Hello, 1 world$!

Helloworld

[lastchance]

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
using namespace std;

string filter( string s, bool banned( char c ) )
{
   s.erase( remove_if( s.begin(), s.end(), banned ), s.end() );
   return s;
}


int main()
{
   string s = "-Hello, 1 world$!";
   auto filterFunction = []( char c ){ return !isalpha( c ); };
   cout << filter( s, filterFunction ) << '\n';
}

[Ganado]

Alternatively, you can build a second string and have it replace the original string. This avoids any index invalidation.

string filter(const string& str)
{
    string out;
    for (char ch : str)
    {
        if (/* keep ch */)
        {
            out += ch;
        }
    }
    return out;
}