What does '&' do in a C++ declaration?


Question

I am a C guy and I'm trying to understand some C++ code. I have the following function declaration:

int foo(const string &myname) {
  cout << "called foo for: " << myname << endl;
  return 0;
}

How does the function signature differ from the equivalent C:

int foo(const char *myname)

Is there a difference between using string *myname vs string &myname? What is the difference between & in C++ and * in C to indicate pointers?

Similarly:

const string &GetMethodName() { ... }

What is the & doing here? Is there some website that explains how & is used differently in C vs C++?

1
36
10/18/2013 4:43:02 AM

Accepted Answer

The "&" denotes a reference instead of a pointer to an object (In your case a constant reference).

The advantage of having a function such as

foo(string const& myname) 

over

foo(string const* myname)

is that in the former case you are guaranteed that myname is non-null, since C++ does not allow NULL references. Since you are passing by reference, the object is not copied, just like if you were passing a pointer.

Your second example:

const string &GetMethodName() { ... }

Would allow you to return a constant reference to, for example, a member variable. This is useful if you do not wish a copy to be returned, and again be guaranteed that the value returned is non-null. As an example, the following allows you direct, read-only access:

class A
{
  public:
  int bar() const {return someValue;}
  //Big, expensive to copy class
}

class B
{
public:
 A const& getA() { return mA;}
private:
 A mA;
}
void someFunction()
{
 B b = B();
 //Access A, ability to call const functions on A
 //No need to check for null, since reference is guaranteed to be valid.
 int value = b.getA().bar(); 
}

You have to of course be careful to not return invalid references. Compilers will happily compile the following (depending on your warning level and how you treat warnings)

int const& foo() 
{
 int a;

 //This is very bad, returning reference to something on the stack. This will
 //crash at runtime.
 return a; 
}

Basically, it is your responsibility to ensure that whatever you are returning a reference to is actually valid.

32
12/22/2009 12:04:55 AM

Here, & is not used as an operator. As part of function or variable declarations, & denotes a reference. The C++ FAQ Lite has a pretty nifty chapter on references.


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