const_reference of front and back

#include <list>
#include <iostream>

int main()
{
    char a[] = {'o', 'm', 'g', 'w', 't', 'f'};
    const std::list<char> letters (a, a+6);

    if (!letters.empty()) {
        std::cout << "The first character is: " << letters.front() << '\n';
    }
}
for example when we declare a const list and reference its front or back, we need the const_reference version of these functions according to the standard.