What is the difference between references and normal variable handles in C++?

Question

If C++, if I write:

int i = 0;
int& r = i;

then are i and r exactly equivalent?

Answer 1

You are writing definitions here, with initializations. That means that you're refering to code like this:

void foo() {
    int i = 0;
    int& r = i;
}

but not

class bar {
    int m_i;
    int& m_r;
    bar() : i(0), r(i) { }
};

The distinction matters. For instance, you can talk of the effects that m_i and m_r have on sizeof(bar) but there's no equivalent sizeof(foo).

Now, when it comes to using i and r, you can distinguish a few different situations:

• Reading, i.e. int anotherInt = r;
• Writing, i.e. r = 5
• Passing to a function taking an int, i.e. void baz(int); baz(r);
• Passing to a function taking an int&, i.e. void baz(int&); baz(r);
• Template argument deduction, i.e. template void baz(T); baz(r);
• As the argument of sizeof, i.e. sizeof(r)

In these cases, they're identical. But there is one very important distinction:

std::string s = std::string("hello");
std::string const& cr = std::string("world");

The reference extends the lifetime of the temporary it's bound to, but the first line makes its a copy.