How to properly return std::string (or how to properly use that returned value)

Question

Say you have a class which is a global (e.g. available for the runtime of the app)

class MyClass {
  protected:
    std::string m_Value;
  public:
    MyClass () : m_Value("hello") {}
    std::string value() { return m_Value; }      
};

MyClass v1;

Using the first form gives me odd behavior when I do

printf("value: %s\n", v1.value().c_str());

It looks as though the string disappears from memory before printf can use it. Sometimes it prints value: hello other times it crashes or prints nothing.

If I first copy the string like so

   std::string copiedString = v1.value();
   printf("value: %s\n", copiedString.c_str());

things do work.

Surely there must be a way to avoid doing this with a temporary string.

Edit: So the consensus is to go with a const std::string & return value.

I know everyone says that the original code should be fine but I can tell you that I've seen MSVC 2005 on Windows CE having trouble with it, but only on the CE box. Not the Win32 cross compile.

Answer 1

Your code should work fine. Something else is wrong, that we can't detect from this testcase. Perhaps run your executable through valgrind to search for memory errors.

Answer 2

Well, there's nothing wrong with the code (as I interpret it). It is not optimal and surely not The Right Way (R), you should modify your code like villentehaspam suggests. As it is now, your code makes a copy of the string m_value because you return by value, which is not as good as only returning a const reference.

If you provide a complete code sample that shows the problem, I could help you better.