expected constant expression in vc++

Question

Why does not this code compile with VC++ 15 in visual studio 2008?

Errors: C2057: expected constant expression, C2466: cannot allocate an array of constant size 0.

void foo(int a, int b)
{
double arr[a][b]
...
}

Although it works fine in GCC 4.4. I need compatibility between compilers.

Mike Seymour Mike Seymour · Accepted Answer · 2014-04-18T08:40:50

Why does not this code compile with VC++ 15 in visual studio 2008?

Because C-style variable length arrays are not part of the C++ language. GCC provides them as a non-standard extension; some compilers don't.

I need compatibility between compilers.

Then you'll need a dynamic array, for example:

std::vector< std::vector<double> > arr(a, std::vector<double>(b));

or, if you want all the elements to be contiguous as they would be in a 2-dimensional array:

std::vector<double> arr(a*b);

with appropriate arithmetic to calculate the indexes when you access it.