beginner's question on slicing

[subramanian100in]

In his book, Stroustrup has mentioned that slicing can be a source of
surprises and errors. I am unable to understand it. Kindly give an
example where slicing can lead to surprise and error.

Thanks
V.Subramanian

 

[Zeppe]

In his book, Stroustrup has mentioned that slicing can be a source of
surprises and errors. I am unable to understand it. Kindly give an
example where slicing can lead to surprise and error.

ex.

try{
throw std::runtime_error("example error");
}
catch(std::exception e){
std::cout << "there has been an error: " << e.what() << "\n";
}

Regards,

Zepp

 

[Daniel T.]

In his book, Stroustrup has mentioned that slicing can be a source of
surprises and errors. I am unable to understand it. Kindly give an
example where slicing can lead to surprise and error.

struct foo {
virtual int fnc() { return 5; }
};

struct bar : foo {
virtual int fnc() { return 7; }
};

void test( foo y ) {
cout << y.fnc() << '\n';
}

int main() {
bar x;
test( x );
}

What should be output on the screen? One would expect that since we
passed in a 'bar' object to test, it should output '7', but because of
slicing, it outputs '5'.

If 'test()' took a foo& or foo*, or if 'test()' were a template, then
the output would have been '7'.