Define all constants as const references?

Question

Is there a best practice to define a constant? Here is a small example:

#include <vector>

struct mystruct {
    std::vector<double> data;
    mystruct() : data(100000000,0) {};
};

int main(){
    mystruct A;
    int answer = 42;

    const mystruct& use_struct_option_1 = A; // quick
    const mystruct use_struct_option_2 = A; // expensive

    const int& use_answer_option_1 = answer; // good practice?
    const int use_answer_option_2 = answer; // ubiquitous
}

Obviously, initializing use_struct_option_2 that way is expensive because the copy constructor of mystruct is called whereas the way of initializing use_struct_option_1 is quicker. However, does the same apply to types such as integers?

From the code I've been locking at I can tell that

const int use_answer_option_2 = answer;

is much more common than

const int& use_answer_option_1 = answer;

Which one is preferable?

Answer 1

These do different things. For example, in the int case:

answer = 43;
cout << use_answer_option_1 << '\n';     // 43
cout << use_answer_option_2 << '\n';     // 42

In other words, option 2 makes a copy and option 1 doesn't.

Decide whether you want to make a copy or not (ie whether you want to see changes to the original initializer reflected in your reference). The mystruct case is the same.