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STRANGE CODE:

Pointer references

In C, if you want to modify the contents of the pointer rather than what it points to, your function declaration looks like:

void f(int**);

and you’d have to take the address of the pointer when passing it in:

int i = 47;
int* ip = &i;
f(&ip); 

With references in C++, the syntax is cleaner. The function argument becomes a reference to a pointer, and you no longer have to take the address of that pointer. Thus,

//: C11:ReferenceToPointer.cpp
#include <iostream>
using namespace std;

void increment(int*& i) { i++; }

int main() {
  int* i = 0;
  cout << "i = " << i << endl;
  increment(i);
  cout << "i = " << i << endl;
} ///:~

REFERENCE GUIDELINES:

Your normal habit when passing an argument to a function should be to pass by const reference. Although at first this may seem like only an efficiency concern (and you normally don’t want to concern yourself with efficiency tuning while you’re designing and assembling your program), there’s more at stake: as you’ll see in the remainder of the chapter, a copy-constructor is required to pass an object by value, and this isn’t always available.

The efficiency savings can be substantial for such a simple habit: to pass an argument by value requires a constructor and destructor call, but if you’re not going to modify the argument then passing by const reference only needs an address pushed on the stack.

In fact, virtually the only time passing an address isn’t preferable is when you’re going to do such damage to an object that passing by value is the only safe approach (rather than modifying the outside object, something the caller doesn’t usually expect). This is the subject of the next section.