CppCoreGuidelines F.44 在不需要复制且无需返回空对象的时候返回引用 T&

// -*- compile-command: "g++ -std=c++20 code.cpp && ./a"; -*-
#include <iostream>
#include <array>
#include <gsl/gsl>
using namespace std;
using namespace gsl;

class wheel {
    float pressure;
public:
    wheel(float p = 100): pressure(p){}
    friend ostream& operator<<(ostream& o, wheel& w);
};

ostream& operator<<(ostream& o, wheel& w) {
    o << "wheel's pressure is " << w.pressure << endl;
}

class Car
{
    array<wheel, 4> w;
    // ...
public:
    Car(){}
    wheel& get_wheel(int i) { Expects(i < w.size()); return w[i]; }
    // ...
};

void use()
{
    Car c;
    wheel& w0 = c.get_wheel(0); // w0 has the same lifetime as c
    cout << w0;
}

int main()
{
    use();
    return 0;
}