opaque_handle/src/main.cpp

//#define _GNU_SOURCE
#include <cstdlib>
#include <cstring>
#include <cstdio>

#include <unistd.h>

#include "../include/opaque.hh"

static void s_puts(const auto& string)
{
	std::string_view str{string};
#ifdef DIRECT_WRITE
	write(1, str.data(), str.size());
#else
	printf("%.*s", (int)str.size(), str.data());
#endif
}

static void print_view(std::string_view ptr)
{
	//std::string_view ptr{string};

	size_t sz;
#ifdef DIRECT_WRITE	
	// Bypass all caching of the output and immediately invoke `write()` for everything
	if(__builtin_expect((sz = ptr.size()) > 0, true)) {
		write(1, "> ", 2);
		write(1, ptr.data(), sz);
	} else	write(1, "! <null>\n", 8);
	write(1, "\n", 1);
#else
	// All the output from the program is cached and `write()` is only called once at the end.
	if(__builtin_expect((sz = ptr.size()) > 0, true)) {
		printf("> %.*s\n", (int)sz, ptr.data());
	} else puts("! <null>\n");
#endif
}
static inline void print(const std::string& string)
{
	std::string_view sv{string};
	return print_view(sv);
}

enum class moh_hck {
	Template,
	Ref,
	Owned,
};

/// Example using `make_opaque_handle`, like would be done for a C interface
template<moh_hck Create = moh_hck::Owned>
static void use_moh()
{
	std::string* str = new std::string(__PRETTY_FUNCTION__);

	constexpr auto v_print = [](const opaque_handle& v) {
		print(*v);
		{
			auto copied = v;
			s_puts("(copied)");
			print(*copied);
			s_puts("(moved)");
			print(*std::move(copied));
		}
	};

	if constexpr(Create != moh_hck::Owned) {
		constexpr auto _h = [](std::string* ptr, auto op) noexcept {
			if(ptr) {
				switch(op) {
					case opaque_handle_operation::Clone: return new std::string(std::as_const(*ptr));
					case opaque_handle_operation::Delete: delete ptr; break;
				}
			}
			return static_cast<std::string*>(nullptr);
		};

		if constexpr(Create == moh_hck::Template) {
			const opaque_handle v{make_opaque_handle<std::string, _h>(str)};
			v_print(v);
		} else /*if constexpr(Create == moh_hck::Ref)*/ {
			const opaque_handle v{make_opaque_handle<std::string>(str, _h)};
			v_print(v);
		}
	} else {
		const opaque_handle v{make_opaque_handle<std::string>(str, [](std::string* ptr, auto op) noexcept {
			if(ptr) {
				switch(op) {
					case opaque_handle_operation::Clone: return new std::string(std::as_const(*ptr));
					case opaque_handle_operation::Delete: delete ptr; break;
				}
			}
			return static_cast<std::string*>(nullptr);
		})};
		v_print(v);
	}
	s_puts("---\n");
}

// Testing a hypothetical C interface
extern "C" {
	struct c_struct_example;
	struct opaque;

	void black_box(const opaque* __restrict__){}
	
	c_struct_example* cs_create(const c_struct_example* __restrict__ from);
	void cs_free(c_struct_example* p);

	void cs_print(const c_struct_example* p);
}

static void use_c_struct()
{
	using type = c_struct_example;
	auto* allocp = cs_create(nullptr);

	opaque_handle impossible{make_opaque_handle<opaque, opaque_empty_handler<opaque>>(nullptr)};
	black_box(impossible);

	const opaque_handle v{make_opaque_handle<type>(allocp, [](auto* ptr, auto op) noexcept {
		if(ptr) {
			switch(op) {
				case opaque_handle_operation::Clone: return cs_create(ptr);
				case opaque_handle_operation::Delete: cs_free(ptr); break;
			}
		}
		return static_cast<type*>(nullptr);
	})};

	s_puts("> ");
	cs_print(v);
	{
		auto copied = v;
		s_puts("(copied)> ");
		cs_print(copied);
		s_puts("(moved)> ");
		cs_print(std::move(copied));
	}
}

int main()
{
	const 
	opaque_handle v{make_opaque_object_handle<std::string>("Hello world")};

	puts("Auto-downcasting to `const std::string&`:");
	print(*v);
#if 1
	s_puts("\nAttempting invalid safe cast... ");
	try {
		print_view(*v); // Try to cast to std::string_view, not the correct type...
		s_puts(" FAILED\n");
		return 1;
	} catch(opaque_bad_cast&) { // Expected this exception to be thrown
		s_puts(" OK\n");
	} catch(opaque_exception& other) {
		s_puts(" UNEXPECTED (");
		s_puts(other.what());
		s_puts(")\n");
		return 2;
	}
#endif

	puts("\nConverting view directly:");
	print_view(std::string_view{v.cast<std::string, opaque_handle::CAST_UNSAFE /* SAFETY: We know this is the type of `v`'s pointee, so we don't need the type check. */>()}); // Explicit cast needed, *v will choose the wrong type if not.

	puts("\nUsing opaque_make_handle():");
	use_moh<moh_hck::Template>();
	use_moh<moh_hck::Ref>();
	use_moh<moh_hck::Owned>();

	puts("\nUsing a C interface struct:");
	use_c_struct();
	return 0;
}

extern "C" {
	struct c_struct_example {
		int i,j;
		size_t sz;
		char buf[256];
	};

	c_struct_example* cs_create(const c_struct_example* __restrict__ from)
	{
		auto* ptr = reinterpret_cast<c_struct_example*>(aligned_alloc(alignof(c_struct_example), sizeof(c_struct_example)));
		//memset(ptr, 0, sizeof(c_struct_example);
		if(!from) {
			ptr->i = 10;
			ptr->j = 11;
			snprintf(ptr->buf, 256, "Hello from C interface");
		} else {
			//memcpy(ptr, from, sizeof(c_struct_example));
			*ptr = *from;

			ptr->i +=1;
			ptr->j -=1;
			snprintf(ptr->buf, 256, "Hello from copied C interface (from %p)", from);
		}
		return ptr;
	}

	void cs_print(const c_struct_example* p)
	{
#ifdef DIRECT_WRITE
		char ibuf[512];
		size_t n = snprintf(ibuf, 512, "%s: (%d, %d)", p->buf, p->i, p->j);
		write(1, ibuf, n < sizeof(ibuf) ? n : 511);
		write(1, "\n", 1);
#else
		printf("%s: (%d, %d)\n", p->buf, p->i, p->j);
#endif
	}

	void cs_free(c_struct_example* p)
	{
		free(p);
	}
}