[cfe-dev] Question on constexpr

[Fernando Pelliccioni]

On Sun, Jul 22, 2012 at 9:49 PM, Howard Hinnant <hhinnant at apple.com> wrote:

> I'm playing with this code created by Scott Schurr and presented at C++Now:
>
> http://cppnow.org/session/cpp11-new-tools-for-class-and-library-authors/
>
> #include <cstddef>
> #include <stdexcept>
>
> class str_const
> {
>     const char* const p_;
>     const std::size_t  sz_;
> public:
>     template<std::size_t N>
>         constexpr str_const(const char(&a)[N]) :
>            p_(a), sz_(N-1) {}
>
>     constexpr char operator[](std::size_t n)
>     {
>         return n < sz_ ? p_[n] : throw std::out_of_range("");
>     }
>
>     constexpr std::size_t size() { return sz_; }
> };
>
> template <class T = std::size_t>
> constexpr
> inline
> T
> binary_const(str_const b, std::size_t n = 0, T x = 0)
> {
>     return n == b.size() ? x :
>            b[n] == ',' ? binary_const<T>(b, n+1, x) :
>            b[n] == ' ' ? binary_const<T>(b, n+1, x) :
>            b[n] == '0' ? binary_const<T>(b, n+1, x*2) :
>            b[n] == '1' ? binary_const<T>(b, n+1, x*2+1) :
>            throw std::domain_error("Only '0', '1', ',', and ' ' may be
> used");
> }
>
> int main()
> {
>     constexpr str_const test("Hi Mom!");
>     static_assert(test.size() == 7, "");
>     static_assert(test[6] == '!', "");
>     constexpr unsigned i = binary_const("1111,0000");
>     return i;
> }
>
> $ clang++ -std=c++11 -stdlib=libc++ -O3  test.cpp -S
>
> Everything compiles and runs great (I know, I'm never happy).  But when I
> inspect the assembly of main I see:
>
>         callq   __Z12binary_constImET_9str_constmS0_
>         movl    $240, %eax
>         popq    %rbp
>         ret
>
> Now returning 240 is the right answer.  But why is binary_const being
> called at run time?  Can't this call be elided because we know it is a
> constexpr function (without side effects) and that it is going to return
> the value 240 that we've already computed at compile time?
>
> Howard
>
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>



Hello Howard,

What is the purpose of throwing an exception in a constexpr function?
Or rather... What are the implications of throwing in a constexpr function?

Thanks and Regards,
Fernando.
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