C ++ 14 constexpr std :: array

Question

I study constexpr. I can not understand why the compiler does not consider one of the expressions as constant, because it fits the requirements.

Code:

#include <iostream> #include <cstdlib> #include <array> constexpr int f( int x ) { if ( x < 0 ) x = 0; return ++x; } int main() { typedef double T; typedef std::array<T, 3> C; constexpr C c = {1, 2, 3}; constexpr C::const_reference ref = c.at( 0 ); // error: is not a constexpr static_assert( ref == 1, "ref != 1" ); constexpr int y = -3; //y = f( y ); static_assert( f( y ) == 1, "f( y ) != 1" ); return 0; }

You can compile this link.

In the implementation of stl, with which I work for the array, the at function is declared as constexpr and the array has no constructors at all, the destructor is also not declared, which means the class has a destructor by default, the array itself is initiated, so I don’t know why it does not work.

Interested compilers gcc 5.4.0 and gcc 6.4.0. clang doesn't compile this either.

Answer 1 · 2018-11-17T19:20:17

The concept of "constexpr-link" refers to the properties of the link itself, and not the referenced expression. Constexpr-link assumes that such a link will be tied to an object whose memory location is known at the compilation stage . The situation is almost completely similar to that of constexpr-pointers.

Your local object does not satisfy this requirement.

Declare c as

 static constexpr C c = {1, 2, 3};

and you will be able to link links to it and its sub- constexpr .

(Illustrating with additional examples ...) This code is correct, despite the fact that the link is bound to a regular non-constant variable

 int main() { static int x = 42; constexpr const int &rx = x; }

But such a code is not correct, no matter how much const and constexpr we would put on the variable

 int main() { constexpr int y = 42; constexpr const int &ry = y; }

The value of the constexpr object can be known at compile time.
The position in memory of a constexpr object is not necessarily known at compile time.
(Namely: the position in memory is known only for static objects).
Constexpr-links require precisely the fame of the position in memory.
Your example of the link is completely past the cash register, because there are no attempts to advertise a constexpr link in it.
Apparently, the case is really in the links, but that test worked out, tomorrow if I accomplish my goal, and maybe there will be another mistake, ok?
Simply, I repeat, I tried to re-create the code from memory, and maybe I forgot something.
In general, in the implementation I work with at a non-constant function is called.

C ++ 14 constexpr std :: array

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