Is it ok to return default argument's value by const reference?

Question

Is it ok to return default argument's value by const reference like in the examples below:

https://coliru.stacked-crooked.com/a/ff76e060a007723b

#include <string>  const std::string& foo(const std::string& s = std::string("")) {     return s; }  int main() {     const std::string& s1 = foo();     std::string s2 = foo();      const std::string& s3 = foo("s");     std::string s4 = foo("s"); } 

Answer 1

In your code, both s1 and s3 are dangling references. s2 and s4 are ok.

In the first call, the temporary empty std::string object created from the default argument will be created in the context of the expression containing the call. Therefore, it will die at the end of the definition of s1, which leaves s1 dangling.

In the second call, the temporary std::string object is used to initialize s2, then it dies.

In the third call, the string literal "s" is used to create a temporary std::string object and that also dies at the end of the definition of s3, leaving s3 dangling.

In the fourth call, the temporary std::string object with value "s" is used to initialize s4 and then it dies.

See C++17 [class.temporary]/6.1

A temporary object bound to a reference parameter in a function call (8.2.2) persists until the completion of the full-expression containing the call.