Error Handling

This short tutorial will show you how to handle and report errors from the parser. Schnek uses the C++ exception handling system to process errors. This makes it very easy to report syntax errors and undefined variables.

In the previous tutorial we have seen how to read values from a configuration file. But what happens if there is a syntax error in the configuration file? Let’s modify the file and deliberately introduce an error into the file.

size = 123;
dx = 3.056 + ;
name = "Setup Test";

In the second line of the input file I have added a plus symbol, creating an expression that can’t be interpreted properly. If we run this file through the example code from the previous tutorial the output will look something like this.

terminate called after throwing an instance of 'schnek::ParserError'
Aborted (core dumped)

This is not very informative, but as you can see a ParserError has been thrown. We can catch that error and extract a bit more information from it. The following code shows how this is done.

  pBlock application;

  try
  {
    application = P.parse(in,"File Name");
  }
  catch (ParserError &e)
  {
    std::cerr << "Parse error in " << e.getFilename() << " at line " << e.getLine() << ": "<< e.message << "\n";
    exit(-1);
  }

The ParserError class contains three methods that provide information, getFilename() returns a predefined name of the file. This name can be specified as an optional second argument to the parse() method of the parser. The method getLine() returns the line number on which the error occured and the method getMessage() returns a more detailed error message.

With this modification of the code we now get the much nicer error

Parse error in File Name at line 2: Syntax Error

The ParserError class is thrown when the parser encounters syntax errors or other errors that are related to the structure of the configuration file. Another type of error is thrown when the evaluation of a variable fails. Obviously, this error needs be caught when evaluating the parameters.

  SimulationBlock &mysim = dynamic_cast<SimulationBlock&>(*application);
  try
  {
    mysim.evaluateParameters();
  }
  catch (EvaluationException &e)
  {
    std::cerr << "Evaluation error: " << e.getMessage() << std::endl;
  }

It turns out, however, that this is not enough. Schnek detects constant expressions while parsing the configuration file and evaluates them immediately. This means that an evaluation error can also be thrown by the parser. I recommend surrounding the complete code for reading, parsing and evaluating the configuration file with a single try — catch block.

try
  {
    application = P.parse(in,"File Name");

    SimulationBlock &mysim = dynamic_cast<SimulationBlock&>(*application);
    mysim.evaluateParameters();

    mysim.writeValues();
  }
  catch (ParserError &e)
  {
    std::cerr << "Parse error in " << e.getFilename() << " at line " << e.getLine() << ": "<< e.message << std::endl;
    exit(-1);
  }
  catch (EvaluationException &e)
  {
    std::cerr << "Evaluation Error: " << e.getMessage() << std::endl;
    exit(-1);
  }

Let’s try this with the following input where a string is assigned to the integer size variable.

size = "nonumber";
dx = 3.056;
name = "Setup Test";

When running the code now we get a message that tells us that a type conversion error occured during the evaluation.

Evaluation Error: Could not convert types

Side Note: In the example above the string could not be converted to an integer. The reason was that the content did not represent an integer number. Schnek does perform automatic type conversion where possible. An input file like the following is perfectly valid.

string a = "12";
string b = "34";

size = a+b;

After this size will have a value of 1234.

The code for this tutorial can be found in the Schnek repository here.

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