Add SRB2_ASSERT, srb2::NotNull<T>

Add SRB2_ASSERT, superceding I_Assert

This assertion macro always expands to a call of srb2::do_assert, which
is overloaded with two templates: one which applies if the provided
Level is less than or equal to the SRB2_ASSERTION_LEVEL, and one which
is a no-op. When optimizations are enabled, this will verifiably remove
the evaluation of the expression in all cases, instead of evaluating the
expression and doing nothing with it.

Add srb2::NotNull wrapper utility

This is meant to be used in places where pointers are used as
parameters. It can be used with any pointer-like type, not just raw
pointers. During construction of NotNull, the pointer will be asserted
not-null in debug and paranoia builds, and in release optimizations with
no assertions, the code decays gracefully to standard pointer-passing.

src/cxxutil.hpp

@@ -1,9 +1,12 @@
 #ifndef __SRB2_CXXUTIL_HPP__
 #define __SRB2_CXXUTIL_HPP__
 
+#include <cstdlib>
 #include <type_traits>
 #include <utility>
 
+#include "doomdef.h"
+
 namespace srb2 {
 
 template <class F>
@@ -19,7 +22,8 @@ public:
 	void operator=(Finally&& from) = delete;
 
 	~Finally() noexcept {
-		f_();
+		if (call_)
+			f_();
 	}
 
 private:
@@ -32,6 +36,117 @@ Finally<std::decay_t<F>> finally(F&& f) noexcept {
 	return Finally {std::forward<F>(f)};
 }
 
+struct SourceLocation {
+	const char* file_name;
+	unsigned int line_number;
+};
+
+#define SRB2_SOURCE_LOCATION                                                                                           \
+	srb2::SourceLocation { __FILE__, __LINE__ }
+
+#ifndef SRB2_ASSERT_HANDLER
+#define SRB2_ASSERT_HANDLER srb2::IErrorAssertHandler
+#endif
+
+#if !defined(NDEBUG) || defined(PARANOIA)
+// An assertion level of 2 will activate all invocations of the SRB2_ASSERT macro
+#define SRB2_ASSERTION_LEVEL 2
+#else
+// The minimum assertion level is 1
+#define SRB2_ASSERTION_LEVEL 1
+#endif
+
+/// Assert a precondition expression in debug builds.
+#define SRB2_ASSERT(expr) srb2::do_assert<2, SRB2_ASSERT_HANDLER>([&] { return (expr); }, SRB2_SOURCE_LOCATION, #expr)
+
+class IErrorAssertHandler {
+public:
+	static void handle(const SourceLocation& source_location, const char* expression) {
+		I_Error("Assertion failed at %s:%u: %s != true",
+				source_location.file_name,
+				source_location.line_number,
+				expression);
+	}
+};
+
+class NoOpAssertHandler {
+public:
+	static void handle(const SourceLocation& source_location, const char* expression) {}
+};
+
+/// @brief Assert a precondition expression, aborting the application if it fails.
+/// @tparam Expr
+/// @tparam Level the level of this assertion; if it is less than or equal to SRB2_ASSERTION_LEVEL, this overload will
+/// activate.
+/// @param expr a callable which returns a bool
+/// @param source_location a struct containing the source location of the assertion, e.g. SRB2_SOURCE_LOCATION
+/// @param expression the expression evaluated in the expression callable
+/// @param message an optional message to display for the assertion
+template <unsigned int Level, class Handler, class Expr>
+std::enable_if_t<(Level <= SRB2_ASSERTION_LEVEL), void>
+do_assert(const Expr& expr, const SourceLocation& source_location, const char* expression = "") noexcept {
+	static_assert(Level > 0, "level of an assertion must not be 0");
+	if (!expr()) {
+		Handler::handle(source_location, expression);
+		std::abort();
+	}
 }
 
+template <unsigned int Level, class, class Expr>
+std::enable_if_t<(Level > SRB2_ASSERTION_LEVEL), void>
+do_assert(const Expr&, const SourceLocation&, const char* = "") noexcept {
+}
+
+template <typename T>
+class NotNull final {
+	T ptr_;
+
+public:
+	static_assert(std::is_convertible_v<decltype(std::declval<T>() != nullptr), bool>,
+				  "T is not comparable with nullptr_t");
+
+	/// @brief Move-construct from the pointer value U, asserting that it is not null. Allows construction of a
+	/// NotNull<T> from any compatible pointer U, for example with polymorphic classes.
+	template <typename U, typename = std::enable_if_t<std::is_convertible_v<U, T>>>
+	constexpr NotNull(U&& rhs) : ptr_(std::forward<U>(rhs)) {
+		SRB2_ASSERT(ptr_ != nullptr);
+	}
+
+	/// @brief Wrap the pointer type T, asserting that the pointer is not null.
+	template <typename = std::enable_if_t<!std::is_same_v<std::nullptr_t, T>>>
+	constexpr NotNull(T rhs) : ptr_(std::move(rhs)) {
+		SRB2_ASSERT(ptr_ != nullptr);
+	}
+
+	/// @brief Copy construction from NotNull of convertible type U. Only if the incoming pointer is NotNull already.
+	template <typename U, typename = std::enable_if_t<std::is_convertible_v<U, T>>>
+	constexpr NotNull(const NotNull<U>& rhs) : NotNull(rhs.get()) {
+		// Value is guaranteed to be not null by construction; no assertion necessary
+	}
+
+	NotNull(const NotNull& rhs) = default;
+	NotNull& operator=(const NotNull& rhs) = default;
+
+	/// @brief Get the stored pointer.
+	constexpr T get() const { return ptr_; }
+
+	/// @brief Convert to T (the pointer type).
+	constexpr operator T() const { return get(); }
+
+	/// @brief Arrow-dereference to *T (the actual value pointed to).
+	constexpr decltype(auto) operator->() const { return get(); }
+
+	/// @brief Dereference to *T (the actual value pointed to).
+	constexpr decltype(auto) operator*() const { return *get(); }
+
+	// It is not allowed to construct NotNull<T> with nullptr regardless of T.
+	NotNull(std::nullptr_t) = delete;
+	NotNull& operator=(std::nullptr_t) = delete;
+};
+
+template <class T>
+NotNull(T) -> NotNull<T>;
+
+} // namespace srb2
+
 #endif // __SRB2_CXXUTIL_HPP__

src/doomdef.h

@@ -645,6 +645,7 @@ UINT32 quickncasehash (const char *p, size_t n)
 #endif
 
 // An assert-type mechanism.
+// NOTE: USE SRB2_ASSERT FOR C++ CODE INSTEAD
 #ifdef PARANOIA
 #define I_Assert(e) ((e) ? (void)0 : I_Error("assert failed: %s, file %s, line %d", #e, __FILE__, __LINE__))
 #else

src/tests/CMakeLists.txt

@@ -1,3 +1,5 @@
 target_sources(srb2tests PRIVATE
 	boolcompat.cpp
+	notnull.cpp
+	testbase.hpp
 )

src/tests/boolcompat.cpp

@@ -1,3 +1,4 @@
+#include "testbase.hpp"
 #include <catch2/catch_test_macros.hpp>
 
 #include "../doomtype.h"

src/tests/notnull.cpp

@@ -0,0 +1,31 @@
+#include "testbase.hpp"
+#include <catch2/catch_test_macros.hpp>
+
+#include "../cxxutil.hpp"
+
+namespace {
+class A {
+public:
+	virtual bool foo() = 0;
+};
+class B : public A {
+public:
+	virtual bool foo() override final { return true; };
+};
+} // namespace
+
+TEST_CASE("NotNull<int*> is constructible from int*") {
+	int a = 0;
+	REQUIRE(srb2::NotNull(static_cast<int*>(&a)));
+}
+
+TEST_CASE("NotNull<A*> is constructible from B* where B inherits from A") {
+	B b;
+	REQUIRE(srb2::NotNull(static_cast<B*>(&b)));
+}
+
+TEST_CASE("NotNull<A*> dereferences to B& to call foo") {
+	B b;
+	srb2::NotNull<A*> a {static_cast<B*>(&b)};
+	REQUIRE(a->foo());
+}

src/tests/testbase.hpp

@@ -0,0 +1,6 @@
+#ifndef __SRB2_TESTS_TESTBASE_HPP__
+#define __SRB2_TESTS_TESTBASE_HPP__
+
+#define SRB2_ASSERT_HANDLER srb2::NoOpAssertHandler
+
+#endif // __SRB2_TESTS_TESTBASE_HPP__