exception – California Sunshine

As a Java/Python developer for about 10 years, in my mind, invoking a member function from a raw nullptr, there should have always exceptions, as the following shows.

Maybe you think why I discuss about this naive question. Since recently I found, it’s may not be true for C++ and it depends on how the C++ class defines.

Take a look at the following code written by C++ and execution results.

Surprisingly, the code was executed correctly! It’s tested by VC 2019, GCC 4.8.4/7.3.

I never noticed this but after more time thinking, you may understand the reason. It’s because there is no member field de-referenced in C++ class of “A”.

Let me explain it simply. The function A::hello() will be renamed (there is a full set of naming convention) and compiled into a kind of a C function. And the parameter of pointer of “this” will be added into the function (not that simple, but you can understanding like that). So A::hello() will be compiled to a function like _ZN1A5helloEv(A* this). When you call “a->hello()”, actually, it looks like “_ZN1A5helloEv(a)”. Calling a C function will not cause segment fault, but de-referencing will.

The following code snippet will give you better understanding.

In OOP, this kind of class “A” is not useful. No member fields means a object has no properties. For this case, you should use static function and invoking the function as a class function.

But this tricky behavior is good for us to understand how C++ language works.

If you have a log system, such as slf4j or log4j, sometimes you want to print the exception’s stack trace into log system (a file, a console or something else). Default behavior of e.printStackTrace() is to print the stack trace into stdout.

The easiest way is to convert the stack trace into string then call logger.debug() to print it into log system.

How to do it? Here is a simple solution to it.

public class StackTracePrinter {

  /**
   * Print exception stack into string.
   * @param e the exception.
   * @return the exception stack.
   */
  public static String printToStr(Exception e) {
    OutputStream output = new OutputStream() {

      private StringBuilder sb = new StringBuilder();

      @Override
      public void write(int b) throws IOException {
        sb.append((char)b);
      }

      /* (non-Javadoc)
       * @see java.lang.Object#toString()
       */
      @Override
      public String toString() {
        return sb.toString();
      }

    };

    PrintStream prtStream = new PrintStream(output);
    e.printStackTrace(prtStream);

    // PrintStream will not flush automatically.
    prtStream.flush();

    return output.toString();
  }

}

public class StackTracePrinter {

/**

* Print exception stack into string.

* @param e the exception.

* @return the exception stack.

public static String printToStr(Exception e) {

OutputStream output = new OutputStream() {

private StringBuilder sb = new StringBuilder();

@Override

public void write(int b) throws IOException {

sb.append((char)b);

}

/* (non-Javadoc)

* @see java.lang.Object#toString()

@Override

public String toString() {

return sb.toString();

}

};

PrintStream prtStream = new PrintStream(output);

e.printStackTrace(prtStream);

// PrintStream will not flush automatically.

prtStream.flush();

return output.toString();

}

This sample also show you the flexability of OutputStream.

Tag: exception

A tricky thing in C++ – Will a member function call by using a raw nullptr crash? Maybe no.

Convert exception stack trace into String