Is constexpr really needed?

Question

I have been looking at the new constexpr feature of C++ and I do not fully understand the need for it.

For example, the following code:

Answer 1

constexpr allows the following to work:

#include
using namespace std;

constexpr int n_constexpr() { return 3; }
int n_NOTconstexpr() { return 3; }


template
struct Array { typedef int type[n]; };

typedef Array::type vec_t1;
typedef Array::type vec_t2; // fails because it's not a constant-expression

static const int s_maxSize = n_NOTconstexpr();
typedef Array::type vec_t3; // fails because it's not a constant-expression

template arguments really do need to be constant-expressions. The only reason your example works is because of Variable Length Arrays (VLAs) - a feature that is not in standard C++, but might be in many compilers as an extension.

A more interesting question might be: Why not put constexpr on every (const) function? Does it do any harm!?