C++ Returning a Reference

Question

Consider the following code where I am returning double& and a string&. The thing works fine in the case of a double but not in the case of a string. Why is this difference in the behavior?

In both the cases compiler does not even throws the Warning: returning address of local variable or temporary as I am returning a reference.

#include <iostream>
#include <string>
using namespace std;


double &getDouble(){
    double h = 46.5;
    double &refD = h;
    return refD;
}

string &getString(){
    string str = "Devil Jin";
    string &refStr = str;
    return refStr;
}

int main(){
    double d = getDouble();
    cout << "Double = " << d << endl;

    string str = getString();
    cout << "String = " << str.c_str() << endl;


    return 0;
}

Output:

$ ./a.exe
Double = 46.5
String =

Answer 1

You should never return a reference to a local variable no matter what the compiler does or does not do. The compiler may be fooled easily. you should not base the correctness of your code on some warning which may not have fired.

The reason it didn't fire here is probably that you're not literally returning a reference to a local variable, you are returning a variable that is a reference to a local variable. The compiler probably doesn't detect this somewhat more complex situation. It only detects things like:

string &getString(){
    string str = "Devil Jin";
    return str;
}

The case of the double is simpler because it doesn't involve constructing and destructing a complex object so in this situation the flow control analysis of the compiler probably did a better job at detecting the mistake.

Answer 2

The reference to a double references an location that is still physically in memory though it is no longer be on the stack. You're only getting away with it because the memory hasn't been overwritten yet. Whereas the double is a primitive, the string is an object and has a destructor that may be "clearing" the internal string to a zero length when it falls out of scope. The fact that you aren't getting garbage from your call to c_str seems to support that.

Answer 3

GCC used to have an extension called Named Returns that let you accomplish the same thing, but allocated the space outside the function. Unfortunately it doesn't exist anymore; I'm not sure why they took it out

Answer 4

I had a similar post and the answers there were very helpful.

Also, check out the comments

http://stackoverflow.com/questions/2612709/why-does-this-object-wonk-out-get-deleted