Getting the number of elements in std::array at compile time

Question

Is the following a valid C++ code, and why not?

std::array a1;
std::array a2;

It doesn\'t compile

Answer 1

std::array::size is actually required to be constexpr per § 23.3.2.1 of the C++11 standard:

23.3.2.4 array::size [array.size]  
template  constexpr size_type array::size() noexcept;  
Returns: N

I'm guessing this just slipped past whoever implemented it in GCC.

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After testing, this works:

std::array a1;
std::array a2;

This may actually have something to do with std::string not being a valid constexpr type to make compile-time instances of, whereas int is.