问题
Are the C++, C# or Java languages context-free or context-sensitive?
回答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!
来源：https://stackoverflow.com/questions/9652436/are-the-grammars-of-modern-programming-languages-context-free-or-context-sensiti