用const， - int vs float初始化一个constexpr

我想知道为什么整数 ii 在编译时初始化，而不是float ff 这里：

I'm wondering why the integer ii is initiallized at compile time, but not the float ff here:

int main() {
  const int i = 1;
  constexpr int ii = i;

  const float f = 1.0;
  constexpr float ff = f;
 }

这是当我尝试编译时会发生什么：

This is what happens when I try to compile:

> g++ -std=c++11 test.cc
test.cc: In function ‘int main()’:
test.cc:6:24: error: the value of ‘f’ is not usable in a constant expression
   constexpr float ff = f;
                        ^
test.cc:5:15: note: ‘f’ was not declared ‘constexpr’
   const float f = 1.0;