Based on C++ standard (standard version >= C++11), there are two top() methods available for std::stack template class.

reference top();
const_reference top() const;

What’s the difference between the two methods? and how do I know which one I’m calling? For example,

std::stack<int> si;
si.push(4);
si.top(); // which method is this?

I wrote something similar:

struct S {
    int x {5};
    int &foo();
    const int &foo() const;
};
const int &S::foo() const { fmt::println("const int &"); return x;}
int &S::foo() { fmt::println("int &"); return x;}

Every time I call .foo(), int &foo() is picked.