dynamic_cast in assert Causing Error

Question

I'm using the antiquated Visual Studio 2008 (let me save you the trouble "there's your problem".) This seems to be a problem with Visual Studio: http://rextester.com/XKFR77690 This seems to be a problem with the assert macro: http://ideone.com/bhxMi0

Given these structs:

struct base { virtual ~base() {} };

template <typename T>
struct Foo : base { T foo; };

I can do this:

base* test = new Foo<pair<int, int>>;

if(dynamic_cast<Foo<pair<int, int>>*>(test) != NULL) cout << "hello world\n";

But when I use the exact same code as is in the if-statement in an assert: assert(dynamic_cast<Foo<pair<int, int>>*>(test) != NULL) I get an error:

warning C4002: too many actual parameters for macro assert
error C2143: syntax error : missing ',' before ')'

Incidentally I can fix this by using a C-style cast: assert((Foo<pair<int, int>>*)(test) != NULL) But I think that the C-Style cast will do a static_cast not a dynamic_cast which I don't want.

Answer 1

Yup: macros treat top-level commas as argument separators. The simplest fix is to put parentheses around the offending code:

assert((dynamic_cast<Foo<pair<int, int>>*>(test)) != NULL)

or, if you prefer, parentheses around the entire content:

assert((dynamic_cast<Foo<pair<int, int>>*>(test) != NULL))

The reason the C-style cast in the question compiles is not that it's a C-style cast, but that it puts the template code in parentheses, so the comma is no longer at the outermost level.