First face-to-face with Undefined Behavior in C++

Today I spent way too long debugging my toy compiler.

The tokenizer seemed to work perfectly. GDB showed the expected tokens:

exit
69
;

Yet the generated assembly file was empty.

I checked:

• the parser
• the code generator
• the file writing
• the assembler
• the linker

Everything looked fine. The culprit?

size_t m_ind;

I had forgotten to initialize it. It should have been:

size_t m_ind = 0;

or initialized in the constructor.

The funny part is that tokenize() appeared to work. That's the trap with undefined behavior in C++.

An uninitialized variable doesn't always crash your program. Sometimes it "works" just long enough to convince you the bug is somewhere else.

Today's reminder:

• If you're seeing impossible behavior, suspect undefined behavior.
• DO Initialize your member variables.
• Never trust code just because it worked once.