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References

Overview

• What is a reference?

• Review passing references to functions

• const and references

• Reference variables in range-based for loops

• Potential reference pitfalls

• Raw vs. Smart pointers

What is a Reference?

• An alias for a variable

• Must be initialized to a variable when declared

• Cannot be null

• Once initialized cannot be made to refer to a different variable

• Very useful as function parameters

• Might be helpful to think of a reference as a constant pointer that is automatically dereferenced

Usage

• Using references in a range-based for loop

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  vector<string> names{"Jack", "Sunny", "Yena"};
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  for (auto name : names)
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      name = "Hello";         // Changes the copy
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  for (auto name: names)
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      cout << name << endl;   // Jack Sunny Yena
  

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  vector<string> names{"Jack", "Sunny", "Yena"};
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  for (auto &name : names)
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      name = "Hello";         // Changes the actual
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  for (auto name: names)
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      cout << name << endl;   // Hello Hello Hello
  

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  vector<string> names{"Jack", "Sunny", "Yena"};
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  for (auto const &name : names)
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      name = "Hello";         // Compiler error
  

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  vector<string> names{"Jack", "Sunny", "Yena"};
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  for (auto const &name : names)
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      cout << name << endl;   // Jack Sunny Yena
  

• Passing reference to functions

L-values & R-values

L-values

• Values that have names and are addressable

• Modifiable if they are not constants

• Example

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  int x{100};     // x is an l-value
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  x = 1000;
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  x = 1000 + 20;
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  string name;    // name is an l-value
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  name = "Jack";
  

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  100 = x;            // Error: 100 is NOT an l-value
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  (1000 + 20) = x;    // Error: (1000 + 20) is NOT an l-value
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  string name;
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  name = "Jack";
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  "Jack" = name;      // Error: "Jack" is NOT an l-value
  

R-values

• A value that's not an l-value

  • On the right-hand side of an assignment expression

  • A literal

  • A temporary which is intended to be non-modifiable

• Non-addressable and non-assignable

• Example

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  int x{100};         // 100 is an r-value
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  int y = x + 200;    // (x + 200) is an r-value
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  string name;
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  name = "Jack";      // "Jack" is an r-value
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  int max_num = max(20, 30);  // max(20, 30) is an r-value
  

• R-values can be assigned to L-values

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  int x{100};
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  int y{0};
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  y = 100;    // R-value 100 assigned to l-value y
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  x = x + y;  // R-value (x + y) assgiend to l-value x
  

L-value References

• The references we've used are l-value references because we are referencing l-values.

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  int x{100};
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  int &ref1 = x;      // ref1 is reference to l-value
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  ref1 = 1000;
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  int &ref2 = 100;    // Error: 100 is an r-value
  

• The same when we pass-by-reference

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  int square(int &n)
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  {
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      return n * n;
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  }
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  int num{10};
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  square(num);    // OK
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  square(5);      // Error: can't reference r-value 5
  

   

Pointer Parameters vs. Reference Parameters

• Pass-by-value

  • When the function does NOT modify the actual parameter

  • When the parameter is small and efficient to copy like simple types (int, char, double, etc.)

• Pass-by-reference using a pointer

  • When the function does modify the actual parameter

  • When the parameter is expensive to copy

  • It's OK for the pointer to contain a nullptr value

• Pass-by-reference using a pointer to const

  • When the function does NOT modify the actual parameter

  • When the parameter is expensive to copy

  • It's OK for the pointer to contain a nullptr value

• Pass-by-reference using a const pointer to const

  • When the function does NOT modify the actual parameter

  • When the parameter is expensive to copy

  • It's OK for the pointer to contain a nullptr value

  • When you don't want to modify the pointer itself

• Pass-by-reference using a reference

  • When the function DOES modify the actual parameter

  • When the parameter is expensive to copy

  • The parameter will never be nullptr

• Pass-by-reference using a const reference

  • When the function does NOT modify the actual parameter

  • When the parameter is expensive to copy

  • The parameter will never be nullptr