am I stumbling into know gotchas?

I think so. What you get might not be C++ code yet, but preprocessor input. Since the preprocessor may add quotes, concatenate some elements, and remove or duplicate code, it might be too early to interpret the data as C++ code. For example my_variable(23) could be turned into "var23="<<var23 by the preprocessor. So your tokenizer would detect and identifier, two parentheses and a number, when there is actually a string a '<<' token (note that << is a single token, not two < in a row) and an identifier in the C++ code.

Hopefully, C++ actually discourages the use of this kind of changes through the preprocessor, and favours the use of Templates instead. So if those recommendation are enforced, you may be safe trying to do whatever it is you are trying to do.

Another issue: you neither tokenize strings nor multiline comments, this would lead to a lot of things being interpreted wrongly. Eg:

/*
#define This is not a directive because inside a comment
*/
" // This is not a comment because inside a string ";
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