Why Does Calling `c_str()` on a Deallocated `std::string` Returned from a Function Seem to Work, But Actually Leads to Undefined Behavior?

Why Returning C String Literals from std::string Functions and Calling c_str() May Lead to Undefined Behavior

In a recent lecture, you may have encountered a fragment of C code that sparked some confusion:

<code class="cpp">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);
}</code>

It's understandable to assume that this code would fail because "return 'it's me!!'" implicitly invokes the std::string constructor with a const char[]. Consequently, the std::string returned from the function gets deallocated immediately when the function returns. However, surprisingly, the code executes without any apparent issues.

Your analysis of the code is accurate. The behavior exhibited here falls under the umbrella of undefined behavior. In such cases, the repercussions can vary widely. In this specific instance, the memory allocated for the string, even though it's technically deallocated, still holds its original data when accessed. This phenomenon frequently occurs because operating systems don't automatically clear out deallocated memory, opting instead to mark it as available for future use.

However, it's crucial to note that handling memory deallocation is not the responsibility of the C language; it's an intricate detail specific to each operating system implementation. From a C perspective, the catch-all "undefined behavior" applies. Therefore, relying on such behavior is strongly discouraged as it can lead to unpredictable and potentially catastrophic results in your code.

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