Is constexpr really needed?

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int vec[s_maxSize]; is actually illegal in the second example, so that is not possible to do in C++. But your first example is perfectly legal C++0x.

So there’s your answer. You can’t actually do what you propose in C++. It can only be done in C++0x with constexpr.

I would also like to point out, that this code also works in C++0x. Doing this in C++ would require some really fancy class templates.

constexpr unsigned int gcd(unsigned int const a, unsigned int const b)
{
   return (a < b) ? gcd(b, a) : ((a % b == 0) ? b : gcd(b, a % b));
}

char vec[gcd(30, 162)];

Of course, in C++0x you still have to use the ternary operator instead of if statements. But, it works and is still a lot easier to understand than the template version you’d be force to use in C++:

template <unsigned int a, unsigned int b>
class gcdT {
 public:
   static unsigned int const value = gcdT<b, a % b>::value;
};

template <unsigned int a>
class gcdT<a, 0> {
 public:
   static unsigned int const value = a;

};

char vec[gcdT<30, 162>::value];

And then, of course, in C++ you’d still have to write the gcd function if you needed to compute things at runtime because the template can’t be used with arguments that vary at runtime. And C++0x would have the additional optimization boost of knowing that the result of the function is completely determined by the passed in arguments, which is a fact that can only be expressed with compiler extensions in C++.

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