Will an Unused, but Constructed, Object - contained in a Library - be optimised away?
2024.11.25 21:31 spacecadetbobby Will an Unused, but Constructed, Object - contained in a Library - be optimised away?
I know that C++ is pretty amazing for optimising away code that is not needed, but one thing I can't seem to find a direct answer to, is will it optimise away objects that are explicitly constructed in an included library, but which are never actually used? I believe the answer should be yes, but I want to make sure.
Take the Given Code example:
Library.h:
#pragma once namespace predefinedmessages { namespace _ { class myMessagesClass { private: const char* mTitle; const char* mBody; public: myMessageClass(const char* title, const char* message) : mTitle(title), mBody(message) {} const char* getTitle() const { return mTitle; } const char* getMessage() const { return mBody; } } } extern myMessageClass successMessage; extern myMessageClass failureMessage; extern myMessageClass waitingMessage; extern myMessageClass thanksForUsingMessage; } #include "Library.h" namespace predefinedmessages { myMessageClass successMessage = myMessageClass("Success!", "The Operation Was Successful!"); myMessageClass failureMessage = myMessageClass("Failed!", "The Operation Has Failed!"); myMessageClass waitingMessage = myMessageClass("Waiting...", "Please Wait..."); myMessageClass thanksForUsingMessage = myMessageClass("Thank You!", "Thank you for using this fully featured, production ready, software!"); } #include #include "Library.h" using namespace predefinedmessages; int main() { bool errorState = false; /* Do things */ std::cout << "\n\n" << waitingMessage.getTitle() << "\n---------------" << waitingMessage.getMessage(); /* Do more things */ if(errorState) { std::cout << "\n\n" << failureMessage.getTitle() << "\n---------------" << failureMessage.getMessage(); } else { std::cout << "\n\n" << successMessage.getTitle() << "\n---------------" << successMessage.getMessage(); } return 0; } What I'm wondering, is with optimisation turned on, will the compiler see that, despite being explicitly constructed, "thanksForUsingMessage" is never actually used, and then not compile it into the binary, as it would do with pretty well anything else?
I feel like the answer is yes, but then again that explicit construction is pretty explicit, and I can't seem to nail down a yay or a nay with my searches so far, and I'd like to know before I commit to the idea I'm working on.
For a bit of context: I'm working on a sort of "universal" library for my Arduino projects (hence using 'const char*' instead of strings), and I have to be careful with memory, which is part of why I want to have these predefined and reusable messages that I can quickly use for a variety of things, like the logger, the LCD display or for error reporting. A lot of them are common to all my projects, so I want the convenience of having them always just ready-to-go in my library, without needing to construct them in every project... If that's possible. However, I also don't want a bunch of unused objects just hanging out on the stack, taking up space and basically doing the opposite of what I'm trying to achieve.
Also: I'm compiling with GCC, using C++ 20 standard, in Visual Studio (and with an extension for the Arduino parts). And, as far as I know and have read, because the Arduino compiler is still just GCC under the hood and follows all the standard optimisation rules of C++, and so I don't need an Arduino specific answer.
Thanks for reading, and I hope someone can help illuminate this a bit more for me.
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