MS Visual Studio Error: Expected Constant Expression

Question

Take the following code.

#include "iostream"
using namespace std;
unsigned power(unsigned b, unsigned e){return e?b*power(b, e-1):1;}//Raises base b to a power e
int main(int argc, char* argv[])
{const unsigned lev=5, len=power(2, lev)-1;
int arr[len]; //Error according to Microsoft visual studio
cout<<"The code worked."<<endl;
}

My Codeblock compiler is fine with the array allocation, but Microsoft visual studio says that line expects a constant expression.

I understand the difference between stack and dynamic allocation from heap. But In this instance, the parameter len is determined statically anyway, at the compile time. I store the value in a variable (instead of directly using 5) only because my test cases use different len each time I run the program.

So is there any way to make visual studio work with it? Or should I resort to dynamic allocation, even though CodeBlock accepts it?

Answer 1

int arr[len];

len is not a compile time constant expression (it needs to be computed with power function, after all).

C ( as opposed to C++ ) allows for arrays whose length is not know until runtime. Those are called "variable length arrays" (VLAs).

As such, your code is not valid C++. It works with GCC because that compiler (by default) extends C++ with some additional "features", VLAs being one of them.

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But In this instance, the parameter len is determined statically anyway, at the compile time.

No, it is initialised to the return value of the power function. C++ (by default) does not allow for "super compilation" (executing arbitrary amount of code which is deemed to have a constant result), thus the function can only be called at runtime.

Starting with C++11 there's the constexpr keyword which allows you to mark certain (restricted) functions such that they will be executed during complie time when their parameters are compile time constants.