msmf14 msmf14 - 1 month ago 5x
C++ Question

Array[n] vs Array[10] - Initializing array with variable vs real number

I am having the following issue with my code:

int n = 10;
double tenorData[n] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

Returns the following error:
error: variable-sized object 'tenorData' may not be initialized

Whereas using double tenorData[10] works without any problems.

Anyone know why?


In C++, variable length arrays are not legal. G++ allows this as an "extension" (because C allows it), so in G++ (without being -pedantic about following the C++ standard), you can do:

int n = 10;
double a[n]; // Legal in g++ (with extensions), illegal in proper C++

If you want a "variable length array" (better called a "dynamically sized array" in C++, since proper variable length arrays aren't allowed), you either have to dynamically allocate memory yourself:

int n = 10;
double* a = new double[n]; // Don't forget to delete [] a; when you're done!

Or, better yet, use a standard container:

int n = 10;
std::vector<double> a(n); // Don't forget to #include <vector>

If you still want a proper array, you can use a constant, not a variable, when creating it:

const int n = 10;
double a[n]; // now valid, since n isn't a variable (it's a compile time constant)

Similarly, if you want to get the size from a function in C++11, you can use a constexpr:

constexpr int n()
    return 10;

double a[n()]; // n() is a compile time constant expression