Assert: a cross platform drop-in + self-contained C++ assertion library

TLDR

#include <boost/assert.hpp>
#include <sstream>

int main()
{
  float min = 0.0f;
  float max = 1.0f;
  float v = 2.0f;
  BOOST_ASSERT_MSG(v > min && v < max, static_cast<std::ostringstream&>(std::ostringstream().flush() << \
                  "invalid value: " << v << ", must be between " << min << " and " << max).str().c_str());
  return 0;
}

vs

#include <ppk_assert.h>

int main()
{
  float min = 0.0f;
  float max = 1.0f;
  float v = 2.0f;
  PPK_ASSERT(v > min && v < max, "invalid value: %f, must be between %f and %f", v, min, max);

  return 0;
}

Now which do you prefer? I know which I prefer.

Just drop ppk_assert.h and ppk_assert.cpp into your build and get started. (see also customizing compilation)

Why?

It all started with the need to provide a meaningful message when assertions fire. There is a well-known hack with standard assert to inject a message next to the expression being tested:

assert(expression && "message");

But it's limited to string literals. I wanted improve on assert() by providing the following features:

• being able to format a message that would also contain the values for different variables around the point of failure
• having different levels of severity
• being able to selectively ignore assertions while debugging
• being able to break into the debugger at the exact point an assertion fires (that is in your own source code, instead of somewhere deep inside assert implementation)
• no memory allocation
• no unused variables warning when assertions are disabled

---

What?

The library is designed to be lightweight would you decide to keep assertions enabled even in release builds (#define PPK_ASSERT_ENABLED 1).

By default, each assertion eats up sizeof(bool) of stack, used to keep track whether the assertion should be ignored for the remaining lifetime of the program. You can disable this feature (see customizing compilation).

Message Formatting

The library provides printf like formatting:

PPK_ASSERT(expression);
PPK_ASSERT(expression, message, ...);

E.g:

PPK_ASSERT(validate(v, min, max), "invalid value: %f, must be between %f and %f", v, min, max);

Levels Of Severity

This library defines different levels of severity:

• PPK_ASSERT_WARNING
• PPK_ASSERT_DEBUG
• PPK_ASSERT_ERROR
• PPK_ASSERT_FATAL

When you use PPK_ASSERT, the severity level is determined by the PPK_ASSERT_DEFAULT_LEVEL preprocessor token.

You can also add your own additional severity levels by using:

PPK_ASSERT_CUSTOM(level, expression);
PPK_ASSERT_CUSTOM(level, expression, message, ...);

Default Assertion Handler

The default handler associates a predefined behavior to each of the different levels:

• WARNING <= level < DEBUG: print the assertion message to stderr
• DEBUG <= level < ERROR: print the assertion message to stderr and prompt the user for action (disabled by default on iOS and Android)
• ERROR <= level < FATAL: throw an AssertionException
• FATAL < level: abort the program

If you know you're going to launch your program from within a login shell session on iOS or Android (e.g. through SSH), define the PPK_ASSERT_DEFAULT_HANDLER_STDIN preprocessor token.

When prompting for user action, the default handler prints the following message on stderr:

`Press (I)gnore / Ignore (F)orever / Ignore (A)ll / (D)ebug / A(b)ort:`

And waits for input on stdin:

• Ignore: ignore the current assertion
• Ignore Forever: remember the file and line where the assertion fired and ignore it for the remaining execution of the program
• Ignore All: ignore all remaining assertions (all files and lines)
• Debug: break into the debugger if attached, otherwise abort() (on Windows, the system will prompt the user to attach a debugger)
• Abort: call abort() immediately

Under the Windows platform, the default handler also uses OutputDebugString and in the case of a GUI application allocates a console upon encountering the first failed assertion.

Under the Android platform, the default handler also sends log messages to the in-kernel log buffer, which can later be accessed through the logcat utility.

The default handler supports optional logging to a file (suggested by @nothings):

• #define PPK_ASSERT_LOG_FILE "/tmp/assert.txt"
• to truncate the log file upon each program invocation, #define PPK_ASSERT_LOG_FILE_TRUNCATE

Providing Your Own Handler

If you want to change the default behavior, e.g. by opening a dialog box or logging assertions to a database, you can provide a custom handler with the following signature:

typedef AssertAction::AssertAction (*AssertHandler)(const char* file,
                                                    int line,
                                                    const char* function,
                                                    const char* expression,
                                                    int level,
                                                    const char* message);

Your handler will be called with the proper information filled and needs to return the action to be performed:

PPK_ASSERT_ACTION_NONE,
PPK_ASSERT_ACTION_ABORT,
PPK_ASSERT_ACTION_BREAK,
PPK_ASSERT_ACTION_IGNORE,
PPK_ASSERT_ACTION_IGNORE_LINE,
PPK_ASSERT_ACTION_IGNORE_ALL,
PPK_ASSERT_ACTION_THROW

To install your custom handler, call:

ppk::assert::implementation::setAssertHandler(customHandler);

Unused Return Values

The library provides PPK_ASSERT_USED that fires an assertion when an unused return value reaches end of scope:

PPK_ASSERT_USED(int) foo();

When calling foo(),

{
  foo();

  // ...

  bar();

  // ...

  baz();
} <- assertion fires, caused by unused `foo()` return value reaching end of scope

Just like PPK_ASSERT, PPK_ASSERT_USED uses PPK_ASSERT_DEFAULT_LEVEL. If you want more control on the severity, use one of:

PPK_ASSERT_USED_WARNING(type)
PPK_ASSERT_USED_DEBUG(type)
PPK_ASSERT_USED_ERROR(type)
PPK_ASSERT_USED_FATAL(type)
PPK_ASSERT_USED_CUSTOM(level, type)

Arguably, unused return values are better of detected by the compiler. For instance GCC and Clang allow you to mark function with attributes:

__attribute__((warn_unused_result)) int foo();

Which will emit the following warning in case the return value is not used:

warning: ignoring return value of function declared with warn_unused_result attribute [-Wunused-result]

However there is no MSVC++ equivalent. Well there is __checkReturn but it supposedly only have effect when running static code analysis and I failed to make it work with Visual Studio 2013 Express. Wrapping PPK_ASSERT_USED around a return type is a cheap way to debug a program where you suspect a function return value is being ignored and shouldn't have been.

Compile-time assertions

PPK_STATIC_ASSERT(expression)
PPK_STATIC_ASSERT(expression, message)

In case of compile-time assertions, the message must be a string literal and can't be formated like with run-time assertions, e.g:

PPK_STATIC_ASSERT(sizeof(foo) > sizeof(bar), "size mismatch");

When compiled with a C++11 capable compiler, PPK_STATIC_ASSERT defers to static_assert. Contrary to static_assert, it's possible to use PPK_STATIC_ASSERT without a message.

Customizing compilation

In order to use PPK_ASSERT in your own project, you just have to bring in the two ppk_assert.h and ppk_assert.cpp files. It's that simple.

You can customize the library's behavior by defining the following macros:

• #define PPK_ASSERT_ENABLED 1 or #define PPK_ASSERT_ENABLED 0: enable or disable assertions, otherwise enabled state is based on NDEBUG preprocessor token being defined
• PPK_ASSERT_DEFAULT_LEVEL: default level to use when using the PPK_ASSERT macro
• PPK_ASSERT_DISABLE_STL: AssertionException won't inherit from std::exception
• PPK_ASSERT_DISABLE_EXCEPTIONS: the library won't throw exceptions on ERROR level but instead rely on a user provided throwException function that will likely abort() the program
• PPK_ASSERT_MESSAGE_BUFFER_SIZE
• PPK_ASSERT_DISABLE_IGNORE_LINE: disables the injection of a static bool variable used to keep track whether the assertion should be ignored for the remaining lifetime of the program
• PPK_ASSERT_DEBUG_BREAK: lets you redefine programmatic breakpoints

If you want to use a different prefix, provide your own header that includes ppk_assert.h and define the following:

// custom prefix
#define ASSERT                PPK_ASSERT
#define ASSERT_WARNING        PPK_ASSERT_WARNING
#define ASSERT_DEBUG          PPK_ASSERT_DEBUG
#define ASSERT_ERROR          PPK_ASSERT_ERROR
#define ASSERT_FATAL          PPK_ASSERT_FATAL
#define ASSERT_CUSTOM         PPK_ASSERT_CUSTOM
#define ASSERT_USED           PPK_ASSERT_USED
#define ASSERT_USED_WARNING   PPK_ASSERT_USED_WARNING
#define ASSERT_USED_DEBUG     PPK_ASSERT_USED_DEBUG
#define ASSERT_USED_ERROR     PPK_ASSERT_USED_ERROR
#define ASSERT_USED_FATAL     PPK_ASSERT_USED_FATAL
#define ASSERT_USED_CUSTOM    PPK_ASSERT_USED_CUSTOM

Compiling for Windows

There is a Visual Studio 2015 solution in the _win-vs14/ folder.

Compiling for Linux or Mac

There is a GNU Make 3.81 MakeFile in the _gnu-make/ folder:

$ make -j -C _gnu-make/

Compiling for Mac

See above if you want to compile from command line. Otherwise there is an Xcode project located in the _mac-xcode/ folder.

Compiling for iOS

There is an Xcode project located in the _ios-xcode/ folder.

If you prefer compiling from command line and deploying to a jailbroken device through SSH, use:

$ make -j -C _gnu-make/ binsubdir=ios CXX="$(xcrun --sdk iphoneos --find clang++) -isysroot $(xcrun --sdk iphoneos --show-sdk-path) -arch armv7 -arch armv7s -arch arm64" CPPFLAGS=-DPPK_ASSERT_DEFAULT_HANDLER_STDIN postbuild="codesign -s 'iPhone Developer'"

Compiling for Android

You will have to install the Android NDK, and point the $NDK_ROOT environment variable to the NDK path: e.g. export NDK_ROOT=/opt/android-ndk (without a trailing / character).

Next, the easy way is to make a standalone Android toolchain with the following command:

$ $NDK_ROOT/build/tools/make_standalone_toolchain.py --arch=arm --install-dir=/tmp/android-toolchain

Now you can compile the self test and self benchmark programs by running:

$ make -j -C _gnu-make/ binsubdir=android CXX=/tmp/android-toolchain/bin/clang++ LDFLAGS='-llog' CPPFLAGS=-DPPK_ASSERT_DEFAULT_HANDLER_STDIN

---

Credits Where It's Due:

This assertion library has been lingering in my pet codebase for years. It has greatly been inspired by Andrei Alexandrescu's CUJ articles:

• Assertions
• Enhancing Assertions

I learnt the PPK_UNUSED trick from Branimir Karadžić.

Finally, __VA_NARG__ has been invented by Laurent Deniau.

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If you find this library useful and decide to use it in your own projects please drop me a line @gpakosz.