c++ - Returning reference to a local variable

Question

Why can this code run successfully in Code::block. The IDB just reports

warning: "reference to local variable ‘tmp’ returned",

but ouput the result "hello world" successfully.

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

const string &getString(const string &s)
{
    string tmp = s;
    return tmp;
}

int main()
{
    string a;
    cout<<getString("hello world")<<endl;
    return 0;
}

Answer 1

Upon leaving a function, all local variables are destroyed. By returning a reference to tmp, you are returning a reference to an object that soon ceases to exist (that is, technically, the address of a memory region whose contents are no longer meaningful).

Accessing such a dangling reference invokes what is called 'undefined behaviour' - and sadly, 'work as usual' is one kind of 'undefined behaviour'. What might happen here is that std::string keeps a small static buffer for small strings (as opposed to large strings, for which it grabs memory from the heap), and upon leaving getString the stack space occupied by this string is not zeroed so it still seems to work.

If you try a debug build, or invoke another function in between (which will effectively overwrite the stack space), it won't work anymore.