When is it better to use const string& versus const string

Question

I understand that it is better to pass const String references as arguments to methods.

However, if I just need to use a String as a constant in the code, is it still better to reference it? Isn't it just an overhead to get a reference to it? I can see this in a number of places in our code base and I'm trying to understand why this would be desirable.

string bar = "test";
const string& data = "FOO_" + bar; //why is this better than const string data?
insert(data)

The function insert takes a reference,

void insert(const string& data)

The same question would apply to any const object being used in the same fashion.

Thanks!

Maybe I'm crazy but I think const string & data = "FOO_" + bar; doesn't work. — QuestionC
– QuestionC, Commented Jul 24, 2015 at 21:49
A const reference rarely has any overhead. The compiler will know to optimize it. — jcai
– jcai, Commented Jul 24, 2015 at 21:54

Youka · Accepted Answer · 2015-07-24 22:11:35Z

1

const string data = "FOO_" + bar;

Creates a temporary and passes her as argument to the copy ctor of data on construction. Creates two string instances, does a copy and destructs one instance.

const string& data = "FOO_" + bar;

Creates a temporary but assigns her to name data, so she isn't temporary anymore. One instance created, not more.

answered Jul 24, 2015 at 22:11

Youka

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Oktalist Over a year ago

The first case may be eligible for RVO, so the result of operator+ may be directly constructed into data, bypassing the copy.

YoungJohn · Accepted Answer · 2015-07-24 21:58:43Z

1

A reference in C++ should be thought of as an alias for an existing variable. You use a reference when you have a variable that already exists that you would like to access by a different alias (name).

This is most common in function calls when you would like to refer to an existing variable (memory location) as an argument to a function. If you do not want to pass a copy to the function, and you instead want to pass an already existing variable to the function under a new name that is when you use a reference.

Similarly, when you want a class to refer to a variable that already exists you use a reference.

A reference 'should' always refer to an existing variable.

answered Jul 24, 2015 at 21:58

YoungJohn

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