c++ - 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(int argc, const char * argv[])
{
    const char* tempString = myFunction().c_str();

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

    return 0;
}

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

As the string returned from the function is not referenced anywhere, it should be deallocated immediately. That was the theory.

However, letting this code run works without problems. Would be curious to hear what you think. Thanks!

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.