Using assert() with a message

Question

Is it possible to write a message on assert error?

For example, using #include <assert.h>, I can only write:

void foo(int a, int b) {
  assert (a != b);
}

However, on an error I want to see the value of a and b. How can I do that?

Answer 1

I usually simply use assert(a != b && "This is my error message"). This works because char* can be converted to bool and that it never will be false (since the address is not 0).

Answer 2

assert is just a preprocessor macro, that if the condition fails calls abort. You can easily make your own with a simple if statement, and if it fails print the values and call abort just like assert:

void foo(int a, int b)
{
    if (a != b)
    {
        std::cerr << "My own assertion failed: a = " << a << ", b = " << b << '\n';
        abort();
    }

    /* ... */
}

---

However, as you are using C++ you should probably thrown an exception instead, probably std::logic_error:

void foo(int a, int b)
{
    if (a != b)
    {
        std::istringstream is;
        is << "Argument mismatch in `foo`: a = " << a << ", b = " << b;
        throw std::logic_error(is.str());
    }

    /* ... */
}

Answer 3

It is possible to write your own assert macro which can break down an expression into subexpressions and print the values of each; but it requires a lot of code to support it, and the standard assert macro doesn't do that on any implementation I know of.

For an example of how to implement such a thing, see the CATCH test framework, in particular the implementation of the CHECK macro.

Alternatively, you could write macros or functions like ASSERT_EQUAL(a,b), giving each subexpression as a separate argument.

Or you might write your own error-handling code without assert

if (a == b) {
    throw std::runtime_error("foo: a(" + std::to_string(a) + ") must not equal b");
}