什么时候使用const string＆与const string比较好

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!

Answer 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.

Answer 2

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.