c++数组长度必须是常量表达式

Question

我看c++ primer 第五版中文版时 看到了书上说数组的长度必须是常量表达式（书上的原话是指的维度，但感觉是翻译的问题，指的确实是长度），然后我试着用变量来写，能通过编译，这里是什么情况，是编译器没有检查，还是语法上本来就合法？ 而且我在上面写了一个函数，返回值是int，没有标注constexpr，也可以作为数组的长度，同求解释。!

Answer 1

Objection @Agree and accept (Escape
You can use this test

int txt_size() {
  int a, b;
  scanf("%d%d", &a, &b);
  return a -b;
}

Visual inspection is that it has been expanded

Answer 2

In VS2012, except int ib[4*7-14], which can be compiled, everything else fails. So the question is, is it really good for CB to extend language features like this?

Answer 3

Because the value of the variable you wrote is determined,
the compiler has already obtained the exact value when allocating memory to the array.

Answer 4

C++ array length cannot be changed, which is why a very large array storage queue is opened.

int q[10000001];  //C++的超大数组

It’s like there is a variable a, array h,

int a=2;
int h[a];   //此时等价于int h[2];
a=5;     //不管怎么变化，

h[4] does not exist at this time, you can try it.

Answer 5

The possible reason is that your compiler also supports the C99 VLA, which is illegal in the sense of the C++ standard. You can try adding flags like -Wall -pedantic and compile again, you should get an error