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

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?

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.