Returning by Value vs Reference in C++

When deciding whether to return by value or by reference, consider these guidelines:

1. If you're returning a local variable or a temporary, you must return by value. Returning a reference to a local variable or a temporary will result in undefined behavior, because the referenced object will be destroyed when the function ends.
2. If you're returning a large object and want to avoid the cost of copying it, consider returning by const reference. This avoids the copy while still preventing the caller from modifying the object.
3. If you want the caller to be able to modify the returned object, and the object will outlive the function call, you can return by non-const reference.

Here's an example of each:

1#include <string>
2
3// Returns by value
4std::string getNameValue() { return "John"; }
5
6// Returns by const reference
7const std::string& getNameConstRef() {
8  static std::string name = "John";
9  return name;
10}
11
12// Returns by non-const reference
13std::string& getNameRef() {
14  static std::string name = "John";
15  return name;
16}
17
18int main() {
19  std::string name1 = getNameValue();
20  const std::string& name2 = getNameConstRef();
21  std::string& name3 = getNameRef();
22
23  // Modifies the static variable in getNameRef
24  name3 = "Steve";
25}

In general, prefer returning by value unless you have a good reason to return by reference.