Query to GPT-4o

Question: can emplace_back always be used where push_back can be used? If yes, given that emplace_back is supposedly more efficient, why not remove push_back, or alias push_back to emplace_back?

Reply

Yes, emplace_back can always be used in place of push_back because emplace_back can perform everything that push_back does and more. However, there are important distinctions between the two, and reasons for keeping both in the C++ Standard Library.

Differences between push_back and emplace_back

  1. push_back:
  2. Inserts a copy (or move) of an existing object into a container. The object must already be constructed, and push_back moves or copies it into the container.

  3. Usage: cpp std::vector<std::string> vec; std::string s = "Hello"; vec.push_back(s); // Copy s into vec vec.push_back("World"); // Move from temporary "World" into vec

  4. emplace_back:

  5. Constructs an object in-place within the container. It forwards the provided arguments to the constructor of the contained object, allowing direct construction without the need for a temporary object or copy/move operations.
  6. Usage: cpp std::vector<std::string> vec; vec.emplace_back("Hello"); // Directly constructs the string "Hello" in-place

Efficiency: Why is emplace_back more efficient?

emplace_back avoids the overhead of constructing a temporary object and then copying/moving it into the container. By constructing the object directly within the container's memory, it can eliminate unnecessary allocations or operations. For example: - With push_back, you might create a temporary object, which is then moved or copied into the container. - With emplace_back, the object is constructed directly within the container, saving both the construction of a temporary and the move/copy operation.

Why Not Remove push_back or Alias It to emplace_back?

Despite emplace_back being more flexible and often more efficient, there are several reasons for keeping both push_back and emplace_back:

  1. Simplicity and Clarity:
  2. push_back is simpler to use when the object is already constructed. It's straightforward and conveys the intent clearly: "I want to insert this existing object into the container." For basic types (like int, float, or simple objects), the performance difference is negligible.

  3. emplace_back, on the other hand, implies construction of a new object within the container. This additional functionality may be unnecessary when the object is already constructed.

  4. Avoiding Misuse of emplace_back:

  5. Using emplace_back might encourage users to pass constructor arguments unnecessarily when simply passing a fully constructed object with push_back is clearer. For example, emplace_back forwards constructor arguments, which can lead to confusion if not used carefully: cpp std::vector<std::string> vec; std::string s = "Hello"; vec.emplace_back(s); // Constructs a string by copying 's', which is unnecessary here

  6. Backward Compatibility:

  7. push_back has been part of C++ since the early days. Removing it or aliasing it would break a lot of existing code that relies on its well-defined behavior. Aliasing push_back to emplace_back could also introduce subtle issues, especially when working with types that have complex constructors.

  8. Intent:

  9. The two functions convey different intent:
  10. Maintaining this distinction helps make code more readable and understandable. Even though emplace_back can technically be used everywhere, it doesn’t necessarily improve clarity in simple cases.

Conclusion

While emplace_back is more powerful and often more efficient than push_back, especially for complex objects, both have their place in C++. The explicitness of push_back for appending already constructed objects improves readability, while emplace_back is valuable for in-place construction without unnecessary copies or moves. Hence, both are retained to serve distinct purposes in terms of clarity, intent, and backward compatibility.

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