为什么全局变量在编译时不知道？

Question

In C++, I know that if a I declared a variable inside a function, this variable is actually considered as auto local variable (destroyed once a function does return ). So it stands to reason that, a local variable cannot appears in a constant expression like an initializer for a constexpr variable, because simply it known at runtime, it needs the function in which it declared to be executed and that's happening only at runtime.

int x { 10 };
constexpr int y { x }; //error: x should be const

int main()
{
    //..
}

My question is, what would happen if this variable is global? So no runtime functions need to be executed in order to know the value of x , because it does not belong to any functions? My question, in other words, when does exactly the compiler knows the value of this variable x

I already know that, if the variable x is const, then x will be a constant expression but why is that?

Answer 1

In your particular example, the compiler can possibly know. It just doesn't have to.

But what if you have

int x { 10 };
someclass trix{};
constexpr int y { x }; //error: x should be const

Now the constructor for trix just could modify x , and the compiler wouldn't know. Especially if trix.cpp is compiled after the main file.