Any way to have the return string alias with the defined name in caller scope?

[marhuum]

How to have the return string alias can be 'seen' with the defined name in caller scope, without being as global nor passing it off ?
illustration is sort of:

using namespace std;
string& first(string& s);

int main(){
  string m("Hello");
  string return_str, result;

  result = funct(m); // or
                        //cout << first(m);
}


string& first(string& s){
	
	return_str=s + " World";
	return	return_str;

}

How to solve such by making use of best C++ features ?

[George P]

It isn't a question of using "best C++ features, " understanding scope is a basic concept.

A couple of ways to solve a scope issue, without globals or passing a pointer/reference into a function for a return:

#include <iostream>
#include <string>

std::string& func(std::string&);

int main()
{
   std::string str("Hello");

   std::string result { func(str) };

   std::cout << result << '\n';
}


std::string& func(std::string& str)
{
   std::string ret_str { str + " World" };

   // this returns a temp string, this leads to undefined behavior
   // very bad, do not do it!
   return ret_str;
}

#include <iostream>
#include <string>

std::string& func(std::string&);

int main()
{
   std::string str("Hello");

   std::cout << func(str) << '\n';
}


std::string& func(std::string& str)
{
   return str += " World";
}

A bit of a refresher on what scope is:
https://www.learncpp.com/cpp-tutorial/4-1a-local-variables-and-local-scope/

[coder777]

Note that returning a local variable as shown in Furry Guy's first example invokes undefined behavior and thus a possible crash. It's simply wrong.

[George P]

My first snippet knowingly fixed part of a scope issue and traded it for a problem with undefined behavior based on scope.

I thought I'd mentioned that earlier, it was part of the post before I hit submit. I see now it didn't take.

[coder777]

@Furry Guy
You may add a comment (at least line 20 of your first example) to make clear that it is an erroneous return. This kind of bugs are hard to detect.

[ICantC]

You just need to remove the return by reference so you get a copy back, don't you?

std::string func(std::string& str)
{
   return str += " World";
}

Additionally, are you allowed to move local variables? I'm guessing not because the memory "stolen" will be on the previous stack and thus invalid once the called function returns?

[George P]

I did try to retroactively add a comment, and the first and 2nd attempt flaked.

My 'net connection tonight/this morning has been acting up.

Not trying to use that as an excuse, though. It was and is my screw-up in the first place.

[MikeyBoy]

You just need to remove the return by reference so you get a copy back, don't you?

I don't understand why you want to return a string at all here. You're passing str by reference, so whatever the calling code passes in will already be modified.

What purpose is there in also returning the modified string?

[marhuum]

I am responsible for all your vexations, so say sorry.. it's just a notion on "modern" programming language, is there one such feature ?
When I wrote first, on mind was friend or some new C++17 breakthrough or whatever useful brilliant idea by the committee...
Thanks all.