Replace With Alphabet Position

Question

Write a function that is given a string, replace every letter with its position in the alphabet. If anything in the text isn't a letter, ignore it and don't return it. a being 1, b being 2, etc.As an example: AlphabetPosition("The sunset sets at twelve o' clock.") Should return "20 8 5 19 21 14 19 5 20 19 5 20 19 1 20 20 23 5 12 22 5 15 3 12 15 3 11" as a string.

std::string alphabet_position(std::string message)
{ 
    message.erase(std::remove_if(message.begin(), message.end(), isspace), message.end());

    std::ostringstream os;
    for (auto const & c : message)
    {
        char c_lower = std::tolower(c);
        if (c_lower < 'a' || c_lower > 'z')  continue;
        int pos = c_lower - 'a' + 1 ;
        os << pos;
        os << ' ';
    }


    return os.str();
}


int main(){ 

  std::string text; 
  std::cout<<"Enter a phrase :"<<std::endl; 
  std::cin>>text; 
  std::string s_out = alphabet_position(text);
  std::cout<<"the alphabet positions :"<<s_out<<std::endl;


    return 0;
}

I am really struggling with this challenge. The Issue here is that when I enter a phrase, the code will give me only the alphabet positions of the letters before the first space. example: Input: The sunset sets at twelve o' clock. Output: 20 8 5