c++11 - static_assert fails when test condition includes constants defined with const? -

i'm reading bjarne stroustrup's book, "the c++ programming language" , found example explaining static_assert. understood static_assert works things can expressed constant expressions. in other words, must not include expression that's meant evaluated @ runtime.

the following example used in book (i did changes in code. don't think should change that'd produced original example code given in book.)

#include <iostream>  using namespace std;  void f (double speed) {     constexpr double c = 299792.468;     const double local_max = 160.0/(60*60);     static_assert(local_max<c,"can't go fast"); }  int main() {         f(3.25);     cout << "reached here!";     return 0; } 

the above gives compile error. here's compiled using ideone: http://ideone.com/c97of5

the exact code book example:

constexpr double c = 299792.458; void f(double speed) {      const double local_max = 160.0/(60∗60);     static_assert(speed<c,"can't go fast"); // yes error     static_assert(local_max<c,"can't go fast");  }  

the compiler not know value of speed @ compile time. makes sense cannot evaluate speed < c @ compile time. hence, compile time error expected when processing line

static_assert(speed<c,"can't go fast"); 

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the language not guarantee floating point expressions evaluated @ compile time. compilers might support that's not relied upon.

even though values of floating point variables "constants" human reader, not evaluated @ compile time. error message compiler link provided makes clear.

static_assert expression not integral constant expression 

you'll have find way comparison using integral expressions. however, seems moot point. suspect, want make sure speed within limit. makes sense run time check.