用c ++ / c中的变量定义数组的长度

Question

I am reading a book saying that in C++ you can't do this:

int array_size = 3;
int array[array_size];

Then I tried it with gcc,but it didn't complain at all(exception warned about unused array ).

Also I read about this question .The 4th answer says that you can use something like this: char someCondition[ condition ]; To me the condition would only be known until runtime,so the whole thing seems really confounding to me.Can anyone help explain this?

Thanks,G

Answer 1

If You are using a C++ compiler it works because most of the C++ compilers provide a compiler extension that supports Variable Length arguments(VLA) .

If You are using a C compiler it works because the standard allows it.

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In C++, VLA are not allowed by the C++ Standard, so any usage of it through compiler extensions will make your code non portable.
C++ provides std::vector or std::array (C++11) which satisfy all the requirements using variable length array or c-style arrays resp and you should use them.

Note that,since C99 standard, VLA's are allowed in C.