How to set array size with const member variable? - c++

Question

I tried to set array size with const member variable like below
class A {
const int SIZE;
A() : SIZE(10) {}
void aaa() { int a[SIZE]; }
};
I cannot build
a[SIZE]
like this expression.
When I use GCC, build success. But when I use VC++ (Windows), I cannot build.
The error message is "'this' cannot be used in a constant expression"
How to set array size with const member variable?

Answer 1

Variable Length Arrays
when I use VC++ (Windows), I cannot build.
An array int a[SIZE] is created on stack with automatic storage duration.
Then this SIZE usually must be determined at compile-time in C++.
But your SIZE is defined without a value as follows:
const int SIZE;
Thus compilers can not know it's value at compile-time and would show compilation error.
So your SIZE is a dynamic variable.
When I use GCC, build success.
...But some C++ compilers support VLA (variable length arrays) which is a C99 addition and allows declaring C-style arrays on stack with a dynamic length.
VC++ does not support C99 and VLA, but GNU compiler supports VLA as an extension even in C90 and C++.
This is the reason why you can compile the above code by GCC without errors.
If you add -pedantic (-pedantic-errors) option to gcc compile command, we can get warnings (errors) for most gcc extensions.
In this case, with this option we should get warning (error) message: ISO C++ forbids variable length array 'a'.
How to fix the current error ?
(1) If you want to use C-style arrays, one way is making SIZE a macro as follows. Then compilers can know it's value at compile-time:
#define SIZE 10
class A
{
public:
A() {}
void aaa() { int a[SIZE]; }
};
(2) Defining the value of SIZE as a static const variable in the class definition, then compilers again can know it's value at compile-time:
class A
{
static constexpr int SIZE = 10;
public:
A() {}
void aaa() { int a[SIZE]; }
};
(3) C++ provides array functionality by std::vector and std::array which might make our code more readable, portable and robust.
I expect that std::array on stack would be more efficient than std::vector in your case, because 10 ints need less memory and std::array is allocated on stack just only once without causing problems.
This is a solution with std::array:
#include <array>
class A
{
static constexpr int SIZE = 10;
public:
A() {}
void aaa() { std::array<int, SIZE> a; }
};

Answer 2

Well, SIZE is probably not a macro because, here, you initialize it with 10 and have it as a variable. If it was a macro, your expression int a[SIZE] would work but you would not need lines 2-4 anyways.
If SIZE is, as here, a normal variable (which includes dynamically creating A with a size parameter, although the input might be from a macro), you should use a vector.
#include <vector>
class A {
const int SIZE;
A() : SIZE(10) {}
void aaa() { std::vector<int> a(SIZE); }
};
Usage of a is now the same.

Answer 3

If you are using an older compiler pre c++11, e.g c++03 you can also do enum {SIZE=10};
class A {
enum {SIZE=10};
void aaa() {int a[SIZE];}
};