constexpr: errors with floating point representation?

Question

I was hoping to convert a constant from degrees to radians (at compile time), so I opted to use a constexpr. However, my program would not compile and so I tried to debug the problem with a few tests. These tests continue to produce errors during compilation.

The problem appears to be correlated with floating point arithmetic when many significant digits are involved.

I tried a quick google search, and I read section 10.4 (Constant Expressions) in Stroustrup's book. Any help would be greatly appreciated. I must be missing something obvious.

Test code:

void testConstantExpressions() {

  constexpr double x0 = 1.0;
  constexpr double y0 = 2.0;
  constexpr double z0 = 4.0;
  constexpr double w0 = x0 / (y0 / z0);
  std::cout << w0 << std::endl;

  constexpr double x1 = 1.0;
  constexpr double y1 = 2.2;
  constexpr double z1 = 4.0;
  constexpr double w1 = x1 / (y1 / z1);
  std::cout << w1 << std::endl;

  constexpr double x2 = 1.0;
  constexpr double y2 = 4.0;
  constexpr double z2 = 2.3;
  constexpr double w2 = x2 / (y2 / z2);
  std::cout << w2 << std::endl;

}

Compiler:

g++ -Wall -c -g -O2 -std=c++11 -frounding-math  main.cpp -o main.o
main.cpp: In function ‘void testConstantExpressions()’:
main.cpp:30:32: error: ‘(1.0e+0 / 5.5000000000000004e-1)’ is not a constant expression
   constexpr double w1 = x1 / (y1 / z1);
                            ^
main.cpp:36:38: error: ‘(4.0e+0 / 2.2999999999999998e+0)’ is not a constant expression
   constexpr double w2 = x2 / (y2 / z2);
                                  ^
make: *** [main.o] Error 1

Answer 1

It's because you specified -frounding-math. You told the compiler you might change rounding mode at runtime, so it can't round at compile time. Did you really mean to?