g ++在static_assert中接受const作为constexpr

Question

When compiling the following code with g++ 4.8.2 and -std=c++11 flag it compiles with no errors:

    constexpr double C = 299792.458;
    const double local_max = 3.5;
    static_assert(local_max < C, "can't go that fast");

When compiling the following code it gives error:

    constexpr double C = 299792.458;
    double x = 3.5;
    const double local_max = x;
    static_assert(local_max < C, "can't go that fast");

Error message:

a.cc: In function 'int main()':

a.cc:6:2: error: non-constant condition for static assertion
static_assert(local_max < C, "can't go that fast");

a.cc:6:2: error: the value of 'local_max' is not usable in a constant expression

a.cc:5:15: note: 'local_max' was not declared 'constexpr' const double local_max = x;

My question is why it does not give error in the first case.

Does it depend on whether a const variable is initialized with a constexpr or not?

Answer 1

You are entirely correct, after

const double local_max = 3.5;

local_max is still not supposed to be a compile-time constant. This is a compiler bug, fixed in GCC 5.1. You can verify this on gcc.godbolt.org , where you will see GCC's error messages, including:

error: the value of 'local_max' is not usable in a constant expression

and

note: 'local_max' was not declared 'constexpr'

Answer 2

The first version is working because the local_max won't be in the memory. Whenever you use local_max, it will be replaced with it's value, which is known at compile time. So you can call a static_assert based on it.

However, in the second version local_max get it's value based on x, which is getting it's value at runtime.

You can't make a static_assert based on a variable which is getting it's value at runtime, because static_assert is a compile time assert.