"FATAL ERROR: SIGABRT" when defining a string constructor in a custom String class

This error occurred

FATAL ERROR: test case CRASHED: SIGABRT - Abort (abnormal termination) signal
::error::Error: Exit with code: 134 and signal: null

when I was testing a string constructor "String::String(const char* other)"

String::String(const char* other) // the constructor I'm talking about
    {
         if (other == nullptr)
         {
              String();
         }
         else if (other != nullptr)
         {
              int count{0};
              for (int i = 0; other[i] != '\0'; ++i)
              {
                  count = count + 1;
              }
              slength = count;
              // delete [] ifmt; // ?
              ifmt = new char [slength + 1];
              for (int i = 0; i < slength; ++i)    
              {
                  ifmt[i] = other[i];
              }
              ifmt[slength] = '\0'; 
         }
    }
    }
String::String()
    {
        slength = 0; 
        ifmt = new char[slength + 1]; 
        ifmt[0] = '\0';
    }

in my custom String class.

private:
            
        int slength;
        char* ifmt;

One of the suggestions I received was to create a new option branch to handle "negative length" and construct an empty string in this case, but I have no idea what "negative length" is.

I have also searched for some case on the Internet but I found I have done the things they suggested but it still doesn’t work.
https://www.geeksforgeeks.org/how-to-create-a-custom-string-class-in-c-with-basic-functionalities/
design a string class constructor c++
custom string exercise in c++

I will be thankful if anyone could give some guidance in this case.

>Solution :

This code snippet

     if (other == nullptr)
     {
          String();
     }

does not make sense.

In this statement

  String();

there is created a temporary object that at once is destroyed.

As a result when a null pointer is passed then the object of the class has uninitialized data members.

Pay attention to that there is a redundant closing brace

    //...
}
}

String::String()