Adding message to assert

Question

Hallo!

I'm looking for a way to add custom messages to assert statements. I found this questions 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".

Answer 1

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.

Answer 2

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.