Add custom messages in assert?

Question

Is there a way to add or edit the message thrown by assert? I\'d like to use something like

assert(a == b, \"A must be equal to B\");

Then

Answer 1

Another option is to reverse the operands and use the comma operator. You need extra parentheses so the comma isn't treated as a delimiter between the arguments:

assert(("A must be equal to B", a == b));

(this was copied from above comments, for better visibility)

Answer 2

As zneak's answer convolutes the code somewhat, a better approach is to merely comment the string text you're talking about. ie.:

assert(a == b); // A must be equal to B

Since the reader of the assert error will look up the file and line anyway from the error message, they will see the full explanation here.

Because, at the end of the day, this:

assert(number_of_frames != 0); // Has frames to update

reads better than this:

assert(number_of_frames != 0 && "Has frames to update");

in terms of human parsing of code ie. readability. Also not a language hack.

Answer 3

For vc, add following code in assert.h,

#define assert2(_Expression, _Msg) (void)( (!!(_Expression)) || (_wassert(_CRT_WIDE(#_Msg), _CRT_WIDE(__FILE__), __LINE__), 0) )

Answer 4

A hack I've seen around is to use the && operator. Since a pointer "is true" if it's non-null, you can do the following without altering the condition:

assert(a == b && "A is not equal to B");

Since assert shows the condition that failed, it will display your message too. If it's not enough, you can write your own myAssert function or macro that will display whatever you want.

Answer 5

You could also just write your own custom assert function. A very simple example:

bool print_if_false(const bool assertion, const char* msg) {
    if(!assertion) {
        // endl to flush
        std::cout << msg << std::endl;
    }
    return assertion;
}

int main()
{
    int i = 0;
    int j = 1;
    assert(print_if_false(i == j, "i and j should be equal"));
    return 0;
}

play with the code.

The assertion reads Assertion print_if_false(i == j, "i and j should be equal").

Answer 6

BOOST_ASSERT_MSG(expre, msg)

http://www.boost.org/doc/libs/1_51_0/libs/utility/assert.html

You could either use that directly or copy Boost's code. Also note Boost assert is header only, so you could just grab that single file if you didn't want to install all of Boost.