Non-type reference parameter can be modified at run-time, does it mean template can be instantiated at run-time?

Question

Recently, I learned about non-type reference parameters like template<auto& t>. Then I find that t can be modified at run-time:

#include <iostream>
template<auto& N>
struct X{
    int operator()() { return N; }
};

int a = 2;
int main()
{
    std::cin >> a; //stdin: 5
    auto temp = X<a>();
    std::cout << temp() << '\n';
}

The output is 5, not 2. Does it mean temp is instantiated at run-time?

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I will try to answer my own question. If anywhere wrong, please correct me, thx! Other answers also welcome!

Answer 1

No, All the standard requires is that the observable behavior be as if the templates were instantiated before the program started to run. .

The reason for the output 5 is that reference type auto& here just means

• N will bind with a when being instantiated and
• the instantiation still occurs at compile-time.

Look at this:

#include <iostream>
template<auto& N>
struct X{
    int operator()() { return N; }
};

int a;
int main()
{
    auto temp = X<a>();
    std::cout << "Compile-time: " << temp() << '\n'; //output 0
    std::cin >> a; //stdin: 5
    std::cout << "Run-time: " << temp() << '\n'; //output 5
}

live demo

Thanks for Guillaume Racicot's comment, the below is wrong.

a is initialized with 0 at compile-time and modified at run-time. N in temp changed from 0(compile-time) to 5(run-time).

Update:

In many implementations, a is stored in bss segment and will be initialized to zero or do not have explicit initialization in the source code by crt0 at runtime.