Adding message to assert

Hallo!

I'm looking for a way to add custom messages to assert statements. I found this questions https://stackoverflow.com/questions/3692954/add-custom-messages-in-assert but the message is static there. I want to do something like this:

assert((0 < x) && (x < 10), std::string("x was ") + myToString(x));

When the assertion fails I want the normal output plus for example "x was 100".

You are out of luck here. The best way is to define your own assert macro.

Basically, it can look like this:

#ifndef NDEBUG
#   define ASSERT(condition, message) \
    do { \
        if (! (condition)) { \
            std::cerr << "Assertion `" #condition "` failed in " << __FILE__ \
                      << " line " << __LINE__ << ": " << message << std::endl; \
            std::terminate(); \
        } \
    } while (false)
#else
#   define ASSERT(condition, message) do { } while (false)
#endif

This will define the ASSERT macro only if the no-debug macro NDEBUG isn’t defined.

Then you’d use it like this:

ASSERT((0 < x) && (x < 10), "x was " << x);

Which is a bit simpler than your usage since you don’t need to stringify "x was " and x explicitly, this is done implicitly by the macro.

There are some old tricks to include messages without writing your own routines:

The first is this:

bool testbool = false;
assert(("this is the time", testbool));

There is also:

bool testbool = false;
assert(testbool && "This is a message");

The first one works, because the inside parens expression result is the value of 'testbool'. The second one works, because the value of the string is going to be non-zero.

A better alternative is to teach the debugger to stop on assert when it fails, then you could examine not only the x value but any other information including call stack. Perhaps, this is what you are really looking for. Sample implementation is mentioned here https://stackoverflow.com/questions/3314314/ways-to-show-your-co-programmers-that-some-methods-are-not-yet-implemented-in-a-c/3316954#3316954

#define ASSERT_WITH_MESSAGE(condition, message) do { \
if (!(condition)) { printf((message)); } \
assert ((condition)); } while(false)

Extending on Kondrad Rudolph's answer:

#include <iostream>

#ifdef NDEBUG
#define assert(condition, message) 0
#else
#define assert(condition, message)\
   (!(condition)) ?\
      (std::cerr << "Assertion failed: (" << #condition << "), "\
      << "function " << __FUNCTION__\
      << ", file " << __FILE__\
      << ", line " << __LINE__ << "."\
      << std::endl << message << std::endl, abort(), 0) : 1
#endif

void foo() {
   int sum = 0;
   assert((sum = 1 + 1) == 3, "got sum of " << sum << ", but expected 3");
}

int main () {
   foo();
}

Output is...

Assertion failed: ((sum = 1 + 1) == 3), function foo, file foo.cpp, line 13.
got sum of 2, but expected 3
zsh: abort      ./a.out

which is similar to what the std::assert macro outputs on my system just with the additional user defined message

For the sake of completeness, I published a drop-in 2 files assert macro implementation in C++:

#include <pempek_assert.h>

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

  return 0;
}

Will prompt you with:

Assertion 'v > min && v < max' failed (DEBUG)
  in file e.cpp, line 8
  function: int main()
  with message: invalid value: 2.000000, must be between 0.000000 and 1.000000

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

Where

• (I)gnore: ignore the current assertion
• Ignore (F)orever: remember the file and line where the assertion fired and ignore it for the remaining execution of the program
• Ignore (A)ll: ignore all remaining assertions (all files and lines)
• (D)ebug: break into the debugger if attached, otherwise abort() (on Windows, the system will prompt the user to attach a debugger)
• A(b)ort: call abort() immediately

You can find out more about it there:

• blog post
• GitHub project

Hope that helps.

Yes, this is possible.

To enable expression like better_assert((0 < x) && (x < 10), std::string("x was ") + myToString(x));, we are supposed to have a corresponding macro in a form of

#define better_assert(EXPRESSION, ... ) ((EXPRESSION) ? \
(void)0 : print_assertion(std::cerr, \
"Assertion failure: ", #EXPRESSION, " in File: ", __FILE__, \ 
" in Line: ", __LINE__ __VA_OPT__(,) __VA_ARGS__))

in which print_assertion is a proxy function to do the assertion. When the EXPRESSION is evaluated false, all the debug information, the __VA_ARGS__, will be dumped to std::cerr. This function takes arbitrary numbers of arguments, thus we should implement a variadic templated function:

template< typename... Args >
void print_assertion(std::ostream& out, Args&&... args)
{
    out.precision( 20 );
    if constexpr( debug_mode )
    {
        (out << ... << args) << std::endl;
        abort();
    }
}

In the previous implementation, the expression (out << ... << args) << std::endl; make use of fold expression in C++17 (https://en.cppreference.com/w/cpp/language/fold); the constant expression debug_mode is related to the compilation options passed, which is can be defined as

#ifdef NDEBUG
    constexpr std::uint_least64_t debug_mode = 0;
#else
    constexpr std::uint_least64_t debug_mode = 1;
#endif

It also worth mentioning that the expression if constexpr( debug_mode ) makes use of constexpr if (https://en.cppreference.com/w/cpp/language/if) imported since C++17.

To wrap everything up, we have:

#ifdef NDEBUG
    constexpr std::uint_least64_t debug_mode = 0;
#else
    constexpr std::uint_least64_t debug_mode = 1;
#endif

template< typename... Args >
void print_assertion(std::ostream& out, Args&&... args)
{
    out.precision( 20 );
    if constexpr( debug_mode )
    {
        (out << ... << args) << std::endl;
        abort();
    }
}
#ifdef better_assert
#undef better_assert
#endif
#define better_assert(EXPRESSION, ... ) ((EXPRESSION) ? (void)0 : print_assertion(std::cerr, "Assertion failure: ",  #EXPRESSION, " in File: ", __FILE__, " in Line: ",  __LINE__ __VA_OPT__(,) __VA_ARGS__))

A typical test case demonstrating its usage can be:

double const a = 3.14159265358979;
double const b = 2.0 * std::asin( 1.0 );
better_assert( a==b, " a is supposed to be equal to b, but now a = ", a, " and b = ", b );

This will produce something error message like:

Assertion failure: a==b in File: test.cc in Line: 9 a is supposed to be equal to b, but now a = 3.1415926535897900074 and b = 3.141592653589793116
[1]    8414 abort (core dumped)  ./test

And the full source code is available in this repo: https://github.com/fengwang/better_assert

To take on Feng Wang answer, in newer versions of C++, anything inline is going to be optimized out. So you can have an inline function in a header file which does all the work.

inline constexpr void NOT_USED()
{
}

template <class T, class ...ARGS>
inline constexpr void NOT_USED(T && first, ARGS && ...args)
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result"
    static_cast<void>(first);
#pragma GCC diagnostic pop
    NOT_USED(args...);
}

template<typename ... ARGS>
void SAFE_ASSERT(bool test_result, ARGS &&... args)
{
#ifdef _DEBUG
    if(test_result)
    {
        (std::cerr << ... << args) << std::endl;
        abort();
    }
#else
    NOT_USED(test_result, args...);
#endif
}

Some comments:

1. If _DEBUG is not defined, the function becomes empty so it can be optimized out 100%

2. If you call the function with a side effect, the non-debug code still works:

    my_assert(c++ < --z, "the #define versions do not behave similarly");
   

   These side effects are clearly visible here. However, if you call a function, it could be really difficult to know whether something happens in the function which was not otherwise expected.

   There are ways to prevent such side effects from happening, though (Example). But all in all, in some cases, you need to call a function for the test and it may have a side effect and therefore needs to not be optimized out in non-debug code.

3. I use abort() because I know that stops the debugger properly, std::terminate() is the C++ way which in modern systems does the same thing, but if std::terminate() doesn't work for your debugger the abort() will.

4. The NOT_USED() is to avoid warnings about unused function parameters (if you don't have that warning, you may end up with expected bugs).

5. I have an implementation in snapdev: see safe_assert.h and not_used.h.

going along with Konrad Rudolf's answer you can do it a bit more concise with

#include <assert.h>
#include <stdio.h>
#define ASSERT(condition,...) assert( \
    condition|| \
    (fprintf(stderr,__VA_ARGS__)&&fprintf(stderr," at %s:%d\n",__FILE__,__LINE__)) \
);

which also works in C,

it works using the general idea from some of the answers to the question you linked, but the macro allows it to be a little more flexible