Developer Handbook

About

The purpose of the Developer Handbook is similar to that of the README. The README should be viewed as a prerequisite to the Developer Handbook. The README should provide information needed to build the project, which might be used by an advanced user or a person trying to build and package the project. The Developer Handbook should focus on information needed by a developer working on the project.

Project Structure

Overview

All components have a platform indicator next to them:

(PG) - PC, GBA
(-G) - GBA
(P-) - PC

Nostalgia
- core - platform abstraction and user I/O (PG)
  - gba - GBA implementation (-G)
  - sdl - SDL2 implementation (P-)
  - qt - Qt implementation, mostly for studio support (P-)
  - userland - common things needed by all non-bare-metal implementations (P-)
  - studio - studio plugin for core (P-)
- player - plays the games (PG)
- studio - makes the games (P-)
- tools - command line tools (P-)
  - pack - packs a studio project directory into an OxFS file (P-)
- world - defines processes map data (PG)
  - studio - studio plugin for world (P-)
deps - project dependencies
- Ox - Library of things useful for portable bare metal and userland code. Not really that external...
  - clargs - Command Line Args processing (P-)
  - claw - Reads and writes Metal or Organic Claw with header to indicate which
  - fs - file system (PG)
  - mc - Metal Claw serialization, builds on model (PG)
  - oc - Organic Claw serialization (wrapper around JsonCpp), builds on model (P-)
  - model - Data structure modelling (PG)
  - std - Standard-ish Library with a lot missing and some things added (PG)
- GbaStartup - GBA assembly startup code, mostly pulled from devkitPro under MPL 2.0 (-G)

Code Base Conventions

Formatting

Indentation is done with tabs.
Alignment is done with spaces.
Opening brackets go on the same line as the thing they are opening for (if, while, for, try, catch, function, etc.)
No space between function parentheses and arguments.
Spaces between arithmetic/bitwise/logical/assignment operands and operators.
Pointer and reference designators should be bound to the identifier name and not the type, unless there is not identifier name.

Write C++, Not C

On the surface, it seems like C++ changes the way we do things from C for no reason, but there are reasons for many of these duplications of functionality. The C++ language designers aren't stupid. Question them, but don't ignore them.

Casting

Do not use C-style casts. C++ casts are more readable, and more explicit about the type of cast being used. Do not use dynamic_cast in code building for the GBA, as RTTI is disabled in GBA builds.

Library Usage

C++ libraries should generally be preferred to C libraries. C libraries are allowed, but pay extra attention.

This example from nostalgia::core demonstrates the type of problems that can arise from idiomatically mixed code.

uint8_t *loadRom(const char *path) {
	auto file = fopen(path, "r");
	if (file) {
		fseek(file, 0, SEEK_END);
		const auto size = ftell(file);
		rewind(file);
		// new can technically throw, though this project considers out-of-memory to be unrecoverable
		auto buff = new uint8_t[size];
		fread(buff, size, 1, file);
		fclose(file);
		return buff;
	} else {
		return nullptr;
	}
}

In practice, that particular example is not something we really care about here, but it does demonstrate that problems can arise when mixing what might be perceived as cool old-school C-style code with lame seemingly over-complicated C++-style code.

Here is another more concrete example observed in another project:

int main() {
	// using malloc does not call the constructor
	std::vector<int> *list = (std::vector<int>*) malloc(sizeof(std::vector<int>));
	doStuff(list);
	// free does not call the destructor, which causes memory leak for array inside list
	free(list);
	return 0;
}

The code base where this was observed actually got away with this for the most part, as the std::vector implementation used evidentally waited until the internal array was needed before initializing and the memory was zeroed out because the allocation occurred early in the program's execution. While the std::vector implementation in queston worked with this code and the memory leak is not noticable because the std::vector was meant to exist for the entire life of the process, other classes likely will not get away with it due to more substantial constructors and more frequent instatiations of the classes in question.

Pointers vs References

Pointers are generally preferred to references. References should be used for optimizing the passing in of parameters and for returning from accessor operators (e.g. T &Vector::operator[](size_t)). As parameters, references should always be const.

Error Handling

Exceptions are clean and nice and gleefully encouraged in userland code running in environments with expansive system resources, but absolutely unacceptable in code running in restrictive bare metal environments. The GBA build has them disabled. Exceptions cause the compiler to generate a great deal of extra code that inflates the size of the binary. The binary size bloat is often cited as one of the main reasons why many embedded developers prefer C to C++.

Instead throwing exceptions, all engine code must return error codes. Nostalgia and Ox both use ox::Error to report errors. ox::Error is a struct that has overloaded operators to behave like an integer error code, plus some extra fields to enhance debugability. If instantiated through the OxError(x) macro, it will also include the file and line of the error. The OxError(x) macro should only be used for the initial instantiation of an ox::Error.

In addition to ox::Error there is also the template ox::ValErr<T>. ox::ValErr simply wraps the type T value in a struct that also includes error information, which allows the returning of a value and an error without resorting to output parameters.

ox::ValErr can be used as follows:

ox::ValErr<int> foo(int i) {
	if (i < 10) {
		return i + 1; // implicitly calls ox::ValErr<T>::ValErr(T)
	}
	return OxError(1); // implicitly calls ox::ValErr<T>::ValErr(ox::Error)
}

int caller1() {
	auto v = foo(argc);
	if (v.error) {
		return 1;
	}
	std::cout << v.value << '\n';
	return 0;
}

int caller2() {
	// it is also possible to capture the value and error in their own variables
	auto [val, err] = foo(argc);
	if (err) {
		return 1;
	}
	std::cout << val << '\n';
	return 0;
}

Lastly, there are two macros available to help in passing ox::Errors back up the call stack, oxReturnError and oxThrowError.

oxReturnError is by far the more helpful of the two. oxReturnError will return an ox::Error if it is not 0 and oxThrowError will throw an ox::Error if it is not 0. Because exceptions are disabled for GBA builds and thus cannot be used in the engine, oxThrowError is only really useful at the boundry between engine libraries and Nostalgia Studio.

void studioCode() {
	auto [val, err] = foo(1);
	oxThrowError(err);
	doStuff(val);
}

ox::Error engineCode() {
	auto [val, err] = foo(1);
	oxReturnError(err);
	doStuff(val);
	return OxError(0);
}

Both macros will also take the ox::ValErr directly:

void studioCode() {
	auto valerr = foo(1);
	oxThrowError(valerr);
	doStuff(valerr.value);
}

ox::Error engineCode() {
	auto valerr = foo(1);
	oxReturnError(valerr);
	doStuff(valerr.value);
	return OxError(0);
}

File I/O

All engine file I/O should go through nostalgia::core::Context, which should go through ox::FileSystem. Similarly, all studio file I/O should go throuh nostalgia::studio::Project, which should go through ox::FileSystem.

ox::FileSystem abstracts away differences between conventional storage devices and ROM.

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