现代编程语言的语法是无上下文还是上下文敏感？

Question

C ++，C＃或Java语言是无上下文还是上下文敏感？

Answer 1

C++ is neither context-free nor context-sensitive, since the template system is Turing-complete and determining whether a piece of C++ code is legal C++ is undecidably hard. For example, I could define a template class that simulates a TM on a string and then creates a constant with value 1 if the machine accepts and 0 if it does not. If I did that, then the following code would be legal iff the TM halted on the given input:

int myArray[TMTemplate</* ... args ... */>::value];

Since if the TM rejects, this creates an array of size 0, which is not allowed.

Neither C# nor Java is context-free, because checking of whether a variable is used correctly and consistently throughout a particular scope is known not to be context-free (the proof is complex and relies on Ogden's lemma ). However, I'm not sure whether or not they are context-sensitive.

Hope this gives a partial answer to your questions!