"Toy Box" for ebe

Since about November 19, 2012 I have been working steadily rewriting the ebe integrated development environment in C++ using the Qt toolkit for the GUI controls.  The new ebe is better in many ways than the older one which was written in Python.

One feature I have been considering was to allow beginners to enter simple expressions and evaluate them the same way that the compiler would do.  I searched for C and C++ interpreters thinking that an interpreter might make a good choice for speedy computations.  During my search I came across a Python program which does live calls to g++ to "interpret" the C++ as it is entered.  I had to wonder about the speed of the compilations.

I started by compiling hello.c and hello.cpp to see how long it took.  On my Core i7 desktop it took about 0.1 seconds for compiling hello.cpp which hello.c took about 0.06 seconds.   I copied hello.c to hello.cpp and it still required 0.06 seconds compiled with g++.  That meant that using printf would be superior for my needs over cout.  Compilations could be done in 0.6 seconds and executing the small program took about 0.002 seconds.  So perhaps I could get 16 compilations per second.

I knew enough about the C++ typeid to figure out the type of an expression, but in order to capture the data precisely I wanted to dump the data for the expression as hexadecimal.  That would allow me to capture floats and doubles exactly as they were computed in the expression.  It seemed a shame to write one program to determine the type of an expression and a second program to dump the value of the expression in hex.  So I kept searching.

Eventually I ran across the g++ typeof operator.  This would allow me to declare a variable of the exact type as an expression.   Consider the declaration below
     typeof((a+b+c)/3.0) x;
This declaration determines that the type of the expression is double and declares x as a double.  With this interesting feature it was easy to write code to declare a variable of the right type to assign the value of an expression and then, using sizeof, I could dump each byte of the variable.

Here you can see the "Toy Box" after defining 3 int variablees and evaluating some expressions.  Line 1 of the lower table shows the original type, format and result for the expression in column 1.  In the others I have used the Format combobox to select an alternative format.  This seems like a useful tool to use when learning a language.  I feel sure that a lot of teachers would enjoy illustrating how C/C++ evaluates expressions using the toybox.  This is far superior to using an interpreter which would probably approximate the syntax and the behavior,  This tool uses the compiler so what you see is what you get.