Why does this work: returning C string literal from std::string function and calling c_str()

Question

We recently had a lecture in college where our professor told us about different things to be careful about when programming in different languages. The following is an example in C++:

std::string myFunction()
{
    return "it's me!!";
}

int main()
{
    const char* tempString = myFunction().c_str();

    char myNewString[100] = "Who is it?? - ";
    strcat(myNewString, tempString);
    printf("The string: %s", myNewString);
}

The idea why this would fail is that return "it's me!!" implicitly calls the std::string constructor with a const char[]. This string gets returned from the function and the function c_str() returns a pointer to the data from the std::string.

Since the string returned from the function is not referenced anywhere, it should be deallocated immediately. However, running this code works without problems. Why is that?

Answer 1

Your analysis is correct. What you have is undefined behaviour. This means pretty much anything can happen. It seems in your case the memory used for the string, although de-allocated, still holds the original contents when you access it. This often happens because the OS does not clear out de-allocated memory. It just marks it as available for future use. This is not something the C++ language has to deal with: it is really an OS implementation detail. As far as C++ is concerned, the catch-all "undefined behaviour" applies.