C++: Argument Passing "passed by reference"

Solution 1:

The ability to pass by reference exists for two reasons:

1. To modify the value of the function arguments
2. To avoid make copies of an object for performance reasons

Example for modifying the argument

void get5and6(int *f, int *s)  // using pointers
{
    *f = 5;
    *s = 6;
}

this can be used as:

int f = 0, s = 0;
get5and6(&f,&s);     // f & s will now be 5 & 6

OR

void get5and6(int &f, int &s)  // using references
{
    f = 5;
    s = 6;
}

this can be used as:

int f = 0, s = 0;
get5and6(f,s);     // f & s will now be 5 & 6

When we pass by reference, we pass the address of the variable. Passing by reference is similar to passing a pointer - only the address is passed in both cases.

For eg:

void SaveGame(GameState& gameState)
{
    gameState.update();
    gameState.saveToFile("save.sav");
}

GameState gs;
SaveGame(gs)

OR

void SaveGame(GameState* gameState)
{
    gameState->update();
    gameState->saveToFile("save.sav");
}

GameState gs;
SaveGame(&gs);

1. The object passed to the function is huge (I would use the pointer variant here so that the caller knows the function might modify the value of the variable)
2. The function could be called many times (eg. in a loop)

Also, read on const references. When it's used, the argument cannot be modified in the function.

Solution 2:

This article helped me a lot.

Please forget about pointers for now. And take this with a grain of salt.

A reference is the object. When you pass by reference, you pass the object.

When you pass by value, you pass a copy of the object; another object. It may have the same state, but it is a different instance; a clone.

So, it may make sense to pass by reference if you:

• need to modify the object inside the function
• do not need (or want) to modify the object, but would like to avoid copying it just to pass it to a function. This would be a const reference.

And it may make sense to pass by value if you:

• want to start from an identical twin, and leave the original twin undisturbed
• do not care about the cost of copying the object (for example, I would not pass an int by reference unless I wanted to modify it).

Here, have a look at this code:

#include<iostream>

struct Foo {
  Foo() { }
  void describe() const {
    std::cout<<"Foo at address "<<this<<std::endl;
  }  
};

void byvalue(Foo foo) {
  std::cout<<"called byvalue"<<std::endl;
  foo.describe();
}

void byreference(Foo& foo) {
  std::cout<<"called byreference"<<std::endl;  
  foo.describe();
}

int main() {
  Foo foo;
  std::cout<<"Original Foo"<<std::endl;
  foo.describe();
  byreference(foo);
  byvalue(foo);
}

And compile it like this: g++ example.cpp

Run it: ./a.out

And check the output (the actual addresses may be different in your computer, but the point will remain):

Original Foo
Foo at address 0x7fff65f77a0f
called byreference
Foo at address 0x7fff65f77a0f
called byvalue
Foo at address 0x7fff65f779f0

Notice how the called byreference address is the same as the Original Foo address (both are 0x7fff65f77a0f). And notice how the called byvalue address is different (it is 0x7fff65f779f0).

Take it up a notch. Modify the code to look as follows:

#include<iostream>
#include<unistd.h> // for sleeping

struct Foo {
  Foo() { }
  Foo(const Foo&) {
    sleep(10); // assume that building from a copy takes TEN seconds!
  }
  void describe() const {
    std::cout<<"Foo at address "<<this<<std::endl;
  }  
};

void byvalue(Foo foo) {
  std::cout<<"called byvalue"<<std::endl;
  foo.describe();
}

void byreference(Foo& foo) {
  std::cout<<"called byreference"<<std::endl;  
  foo.describe();
}

int main() {
  Foo foo;
  std::cout<<"Original Foo"<<std::endl;
  foo.describe();
  byreference(foo);
  byvalue(foo);  
}

Compile it the same way, and notice the output (comments not in the output; included for clarity):

Original Foo
Foo at address 0x7fff64d64a0e
called byreference
Foo at address 0x7fff64d64a0e # this point is reached "immediately"
called byvalue # this point is reached TEN SECONDS later
Foo at address 0x7fff64d64a0f

So, the code is meant to exaggerate the cost of a copy: when you called by reference this cost was NOT incurred. When you called by value you had to wait for ten seconds.

Note: my code was compiled in OS X 10.7.4 using GCC 4.8.1. If you are in windows you may need something different from unitsd.h to make the sleep call work.

Maybe this helps.