Array declaration with const expression

Question

I am trying to declare an array with const value globally as below. But it is giving compilation error saying array size is not a const value.

const int a[] = {1, 2, 3, 4, 5};

int arr[a[1]];

But if I copy the same lines into a function, it is working fine.

Can you please let me know the differences and why it is not working when I tried to declare global array.

Vlad from Moscow · Accepted Answer · 2014-03-04 01:13:50Z

4

If your compiler supports the new specifier constexpr of the C++ 2011 Standard then try

constexpr int a[] = {1, 2, 3, 4, 5};

int arr[a[1]];

As for the compiling your code when it is placed in a function then such a code is not C++ compliant. It is a language extension of the compiler you are using. The size of the array shall be a constant expression known at compile time.

In C you may use Variable Length Arrays (VLA). Some copilers included this feature of the C Standard in C++.

edited Mar 4, 2014 at 1:13

answered Mar 4, 2014 at 1:06

Vlad from Moscow

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2 Comments

kadina Over a year ago

Thanks Vlad. I read some where on the net that in C++ array sizes must be constant expressions, not just constant data. so a[1] is not a const expression. But I didn't understand what is const expression. Does it mean that the expression give const value. If that is the case, a[1] also should give const value as per my understanding. Could you please let me know what is const expression?

Vlad from Moscow Over a year ago

@kadina it is a compound question. You simply should know that objects defined with specifier constexpr are evaluated at compile time and the compiler can use such values in definitions of arrays.

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