When to Use References

Section 10.4 When to Use References

So when should you pass by value, reference, or constant reference? Here is a comparison of the three methods and when you should use them.

Pass by value: func(type name)
- Parameter is a copy of passed data.
  
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- Use when passing a small piece of data (int, double, char). Copying these is no more expensive than making a reference.
  
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- Use when passing a large piece of data and you want a copy.
  
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Pass by reference: func(type& name)
- Parameter is an alias for the passed variable.
  
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- Use when passing a large piece of data (string, vector) and you want the function to modify that data.
  
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Pass by const reference: func(const type& name)
- Parameter is an alias for the passed variable but can’t be used to modify it.
  
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- Passing a large piece of data (string, vector) and you do NOT want the function to modify that data.
  
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Of course, any set of rules will have its exceptions. But the guidelines above describe the preferred method for most situations.

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A final point is that a literal value can’t be passed into a function by reference. The reference needs to point at a storage location, not a value. If there is a function void foo(string& s) the following would cause a compiler error:

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// error: can't pass a literal by reference
foo("hello world");

The error message would be something like cannot bind non-const lvalue. Storing the string into a variable and passing that variable would work:

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string s = "hello world";
foo(s);

Insight 10.4.1.

When in doubt, pass by value. It is the safest option. If you are going to pass by reference, favor using a const reference. Passing off data to another function and letting that function modify the data tends to “hide” what is happening and make it harder to reason about what a chunk of code does. A const reference avoids making extra copies but protects us from unwanted side effects that change data unexpectedly.

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