需要帮助来了解C ++代码

Question

I came across this piece of code in "C++ Templates - The complete Guide". I tried my best to find out if this has been explained somewhere. Apologies if it has already been explained

template<int I> void f(int (&)[24/(4-I)]);
template<int I> void f(int (&)[24/(4+I)]);
int main()
{
    &f<4>; // ERROR: division by zero (SFINAE doesn't apply)
}

Book does say that this won't compile for a specific reason but I do not understand the template syntax and how the function is called with & operator at the beginning.

Appreciate the help.

Harish

Answer 1

In fact, it does compile. You can also call one of these functions, eg the following prints + :

template<int I> void f(int (&)[24/(4-I)]) { std::cout << "-" << std::endl; }
template<int I> void f(int (&)[24/(4+I)]) { std::cout << "+" << std::endl; }

int main()
{
    int a[3];
    f <4>(a);
}

Anyhow, both template functions f expect a reference to an int array, whose length depends on template parameter I . For instance, I picked a[3] because 24/(4+I) = 24/8 = 3 for I=4 .

The function is not called by &f<4> , but only instantiated by taking its address (which actually issues a warning for the unused result).

I guess the book assumes the code won't compile due to the division by zero caused by attempting to instantiate the first overload for I=4 , which is not the case.