Question

I have the following code which compiles with no warnings (-Wall -pedantic) with g++

#include <iostream>
#include <string>

using namespace std;

class Foo
{
public:
    Foo(const std::string& s) : str(s)
    { }

    void print()
    {
        cout << str << endl;
    }

private:
    const std::string& str;
};


class Bar
{
public:

    void stuff()
    {
        Foo o("werd");
        o.print();
    }
};


int main(int argc, char **argv)
{
    Bar b;
    b.stuff();

    return 0;
}

But when I run it, only the newline is printed out. What is going on?

If I were to do this inside stuff:

string temp("snoop");
Foo f(temp);
f.print();

then it works fine!

Answer 1

The reason why this fails is because it essentially compiles to the following under the hood.

Foo o(std::string("wurd"));

In this case the Foo value is taking a reference to a temporary object which is deleted after the constructor completes. Hence it’s holding onto a dead value. The second version works because it’s holding a reference to a local which has a greater lifetime than the Foo instance.

To fix this change the memebr from being a const std::string& to a const std::string.