C++ WCHAR: Cannot allocate an array of constant size 0

Question

I'm trying to create a WCHAR:

LONG bufferSize = foo.bar() + 1;
WCHAR wszBaz[bufferSize];

The compiler issues an error:

error C2057: expected constant expression
error C2466: cannot allocate an array of constant size 0
error C2133: 'wszBaz' unknown size

What am I doing wrong?

UPDATE: I added const but it still gives the same error:

const LONG bufferSize = foo.bar() + 1;
WCHAR wszBaz[bufferSize];

Answer 1

The size of an array is a constant expression. bufferSize is not a constant expression.

Use a vector: std::vector<WCHAR> wszBaz(bufferSize);, or a std::wstring.

Answer 2

You are trying to create a variable sized array on the stack.

The best way to do this is to create the array on the heap as follows:

WCHAR* pszBaz = new WCHAR[bufferSize];

You then need to remember to delete[] it when you have finished with it.

Even easier though is just to use std::wstring ...

Answer 3

Why not just use std::wstring?

If you need to make sure it has a certain number of bytes, you can either do that when you construct it:

std::wstring baz(bufferSize, ' ');

or you can use the reserve method:

std::wstring baz;
baz.reserver(bufferSize);

Answer 4

Array sizes must be constant expression:

const int foo = 10;
WCHAR array1[123]; // ok - 123 is a constant expression
WCHAR array2[foo + 10]; // ok too - the expression is constant
WCHAR array3[bar(123)]; // not ok - it may evaluate to the same thing every time, but function calls aren't seen as constant.

Note that const does not make something a const expression. A const expression is something that is constant at compile time. The compiler is smart enough to figure out that something like 5+5 is a const expression, but isn't smart enough to figure out that foo(5,5) is a const expression -- even if foo(x,y) just returns x+y.

In the next C++ standard (C++0x), you will be able to define functions as const-expressions.