C++ throw syntax

Question

I have a general question regarding the syntax of throwing an object. Consider:

#include <stdio.h>

struct bad { };

void func() {
    throw bad;
}

int main(int, char**) {
    try {
        func();
    } catch(bad) {
        printf("Caught\n");
    }

    return 0;
}

This code does not compile (g++ 4.4.3), as the 'throw' line must be replaced with:

throw bad();

Why is this? If I'm creating a stack-allocated bad, I construct it like so:

bad b;
// I can't use 'bad b();' as it is mistaken for a function prototype

I've consulted Stroustrup's book (and this website), but was unable to find any explanation for what seems to be an inconsistency to me.

Answer 1

throw bad;

doesn't work because bad is a data type, a structure (struct bad). You cannot throw a data type, you need to throw an object, which is an instance of a data type.

You need to do:

bad obj;
throw obj;

because that creates an object obj of bad structure and then throws that object.

Answer 2

You need to throw an instance of the struct.

void func() {
    bad b;
    throw b;
}