How come the compiler thinks this variable isn't constant?

问题

This is my code:

int main()
{
 const int LEN = 5;
 int x[LEN];
}

VS10 says:

error C2057: expected constant expression

error C2466: cannot allocate an array of constant size 0

error C2133: 'x' : unknown size

I even tried the the code in this page and it gives the same problem (I commented the code which gives the error, and uncommented the correct one): http://msdn.microsoft.com/en-us/library/eff825eh%28VS.71%29.aspx

If I was trying a crappy compiler, I would think it's a bug in the compiler, but it's VS2010!

回答1:

You might have compiled your code using .c extension. MS Visual C doesn't support C99. In C89 the size of an array must be a constant expression. const qualified variables are not constants in C. They cannot be used at places where a real constant is required.

Also read this excellent post by AndreyT.

Try saving the file with .cpp extension.

回答2:

As per http://msdn.microsoft.com/en-us/library/3ffb821x.aspx, "Values declared as const that are initialized with constant expressions" are legal in array bounds, so this is valid C++ code.

Thus, that's either a compiler bug or something bizarre coming off a #define somewhere. As sje397's comment suggests, try some name other than LEN for the length? Also, is that actually your entire code, or are headers being #included as well?

Edit to add: Also, the fact that this is valid C++ code of course doesn't matter if you're compiling this as C, as others have noted.

回答3:

You could also use

int main()
{
    enum { LEN = 5 };
    int x[LEN];
}

回答4:

because in this case, I can do :

int main()
{
    const int LEN = 5;
    int* LENptr = (int*)(&LEN);
    *LENptr = 10;
    int x[LEN];
}

which const is only means read-only in this code, not compile-time constant

来源：https://stackoverflow.com/questions/4457978/how-come-the-compiler-thinks-this-variable-isnt-constant

标签

c++

compiler-construction

compiler-errors