add Null terminator on istream error - c++

Question

I am trying to read a text file that has collections of strings into an array of objects, and am having problems with the input. I get an error that goes to istream here
*_Str = _Elem(); // add terminating null character
I don't really know much about how to use strings in C++, so any help would be appreciated.
my code:
char bird_name[MAX_LINE_LENGTH];
char* description =new char [MAX_LINE_LENGTH];
char* sound=new char [MAX_LINE_LENGTH];
int num_states= 0;
char* states[10];
bool valid = true;
char* state_name = new char [MAX_LINE_LENGTH];
for (int j =0; j<10; j++)
states[j]=new char [MAX_LINE_LENGTH];
char *input_filename = argv[1];
ifstream input(input_filename);
if (!input.is_open())
{
cerr << "Invalid filename: " << input_filename << endl;
system("pause");
return 1;
}
input.getline(bird_name, MAX_LINE_LENGTH);
char* state_num = new char [MAX_LINE_LENGTH];
while (strcmp(bird_name, "END") != 0)
{
input.getline(description, MAX_LINE_LENGTH);
consume_newline(input);
input.getline(sound, MAX_LINE_LENGTH);
consume_newline(input);
input.getline(state_num, MAX_LINE_LENGTH);
num_states = int(state_num);
consume_newline(input);
for (int k = 0; k<num_states; k++)
input.getline(states[k], MAX_LINE_LENGTH);
consume_newline(input);
consume_newline(input);
birds[num_birds++] = new Bird(bird_name, description, sound, num_states, states);
//birds[num_birds]->display();
input.getline(bird_name, MAX_LINE_LENGTH);
}

Answer 1

The offending code you mention, …
*_Str = _Elem(); // add terminating null character
is presumably from some standard library source code file.
Note that in your own code you should not use identifiers starting with underscore followed by uppercase, since they are reserved for the implementation (such as the code above).
The comment indicates that things go awry when the standard lib code has read a complete line of input into the buffer, and is trying to add a terminating null-byte.
That in turn indicates that the buffer is too small, or that the buffer pointer handed to the standard library code, is not even valid.
I am unable to find that in the code that you’re showing. And I suspect that the code you’re showing is not the code where the problem manifests. Please note that for the future: if at all possible, post complete code that you have tested one millisecond ago…
Anyway, it’s not necessary to know exactly where and what goes wrong (in detail) in order to fix things. You can just employ an “Alexandrian solution”. That expression refers to Alexander the Great who, when he could not find any rope end to start untying a really Bad Knot™, just sliced it in two with his sword.
So consider your declaration …
char* description =new char [MAX_LINE_LENGTH];
Now the first obvious thing that is wrong with that, glaring us in the face, is the use of an ALL UPPERCASE identifier. Reserve that for macros. Then it becomes …
char* description =new char [max_line_length];
Second, using a raw pointer, and raw new, is generally just Bad™. So get rid of that. Then it looks like …
char description[max_line_length];
Third, using a raw array like that is often a good solution, but it turns out that this one is being used for a variable length string. And for that usage, it is just Bad™. Instead use an object of some string class, such as the standard library’s std::string:
std::string description;
You need to include the [string] header for that, i.e. #include <string>.
Fourth, this variable is only used inside the loop, so move the declaration inside the loop!
Fifth, with std::string, you need to change the getline call, currently …
input.getline(description, MAX_LINE_LENGTH);
to use the freestanding getline function from the [string] header, namely …
std::getline( input, description );
Sixth, there is no error checking on input operations. You need to add error checking and error handling. Assuming that input as a std::istream, then you can check input.fail(); it’s true if some input operation has failed.
Sevent… Oh there should logically be seventh point here, since seven is a much more pleasing number than six. However, I have nothing to say that will fit into this seventh point.
Cheers & hth.,