C++, MyString class with operators overloading

Question

Im getting a warning when I compile my code.. The warning is: reference to local variable 'str' returned [enabled by default] I don't whats the problem or what I'm doing wrong.. This is my code...

MyString& operator+(MyString &a){
    char *tmp=new char[strlen(szArr)+strlen(a.szArr)+1];
    strcpy(tmp, szArr);
    strcat(tmp, a.szArr);
    MyString str(tmp);
    delete tmp;
    return str;
}
MyString& operator+(char *s){
    if(s)
        return *this;
    char *tmp=new char[strlen(szArr)+strlen(s)+1];
    strcpy(tmp, szArr);
    strcat(tmp, s);
    MyString str(tmp);
    delete tmp;
    return str;
}

In both Im getting this warning.. I don't know why is complaining that Im returning the object..

possible duplicate of [Returning the address of local or temporary variable](http://stackoverflow.com/questions/2744264/returning-the-address-of-local-or-temporary-variable) — dyp, Aug 12 '13 at 18:58

score 3 · Accepted Answer · answered Aug 12 '13 at 18:58

3

You are returning a reference to a local (stack) variable that will be out of scope when the function returns. The object will be invalid so the compiler is warning you not to do this. Make sure you have defined a copy constructor and return the object by value (remove the & from the return type).

answered Aug 12 '13 at 18:58

edtheprogrammerguy

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You can return it by value (remove the `&`) which will return a copy of the object. – edtheprogrammerguy Aug 12 '13 at 18:59
Great. Please accept the answer if it is what you were looking for. – edtheprogrammerguy Aug 12 '13 at 19:07
I will, Im just waiting the time limit goes by.. they dont let me accept it.. :( – JV17 Aug 12 '13 at 19:09

score 1 · Answer 2 · answered Aug 12 '13 at 18:58

If you are returning a new object rather than one that already exists, you should not be returning a reference. Remove the reference token. (The object you have constructed will be destructed when the function returns, so the reference will target an invalid object. Returning a non-reference means that the object will be copied/moved instead, which is what you want. Optimizing compilers will elide the copy anyway.)

Also, since you don't modify the arguments, they should be declared const so that const objects (or string literals) can be passed in.

MyString& operator+(MyString &a);
MyString& operator+(char *s);

Should be:

MyString operator+(MyString const &a);
MyString operator+(char const *s);

Note that this logic is backwards:

if(s)
    return *this;

This will no-op if the string is not null. I presume that you meant this:

if(!s)
    return *this;

score 0 · Answer 3 · answered Aug 12 '13 at 18:58

You are creating a temporary MyString str(tmp); variable inside your function, and then you are trying to return a reference to the memory of that variable, which has gone out of scope.

I'm not sure that this is the exact reason for your error message, but it is a problem you will have whenever you try to use that return value for anything.

score 0 · Answer 4 · answered Aug 12 '13 at 18:59

The error is exactly what the compiler is telling you: You are returning a reference to a local variable. Local variables are stored at the stack, and when the function ends its local variables go out of scope, so returning a reference to a local variable has undefined behaviour.
You have to use return by value.

score 0 · Answer 5 · answered Aug 12 '13 at 19:10

0

Another suggestion: you could use the operator form with two function parameters and keep the return-by-reference.

Have a look here for an example

answered Aug 12 '13 at 19:10

DaClown

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C++, MyString class with operators overloading

5 Answers5