二维数组中的预期常数

Question

double rainPerMonth(const int YEARS)
{
   int monthYear[MONTHS][YEARS];
   // ...
}

Visual Studio shows a squiggly line underneath the array declaration, saying that YEARS must be a constant when I'm creating the array. Is this an IDE issue because the variable has yet to be initialized, or am I writing this incorrectly?

MONTHS is already declared globally.

Answer 1

An array size must be a constant expression - that is, a value known at compile time. (Some compilers offer C-style variable-length arrays as a non-standard extension, but I don't think Visual C++ does. Even if it does, it's better not to rely on such extensions.)

A function argument isn't known at compile time, so can't be used as an array size. Your best option is here is probably

std::vector<std::array<int, MONTHS>> monthYear(YEARS);

Answer 2

In C++, an array must be sized at compile time. What you are attempting to do is declare one that is sized at runtime. In the function you've declared, YEARS is only constant within the scope of the function. You could call it rainPerMonth(someInt); where someInt is the result of some user input (which shows you that the result is not a compile-time constant).

Variable Length Arrays are an extension to C, but not C++. To do what you want, you can use dynamic memory, or a std::vector .

Answer 3

I think your problem lies in the fact that C++ wants a constant in the sense of compile-time constant to create your variable monthYear . If you pass it as a function, it need not be known at compile time? For example:

const int x=2;
const int y=3;

char xyChoice;
std::cin >> xyChoice;

if (xyChoice == 'x')
rainPerMonth(x);
else
rainPerMonth(y);

I'm unsure, but it seems to me like this would give you a constant int being passed to your function, but the compiler wouldn't know what size to create an array for before runtime?