Calling a function in main

Question

I\'m just learning C++ and I have a little code here:

using namespace std;

int main()
{
    cout<<\"This program will calculate the weight of         


        

Answer 1

This is a function declaration:

double moon_g();

this won't call a function, and if you did have it correct, which means adding two parameters since that is how you define it below:

moon_g( a, b ) ;

it would not work because you either need to move the definition of moon_g before main or add a forward declaration before main like this:

double moon_g (double a, double b) ;

Although it seems like a and b are not inputs but values you want to return back to main then you would need to use references and it would need to be declared and defined like this:

double moon_g (double &a, double &b) ;
                      ^          ^

A useful thread to read especially if you are starting out would be What is the difference between a definition and a declaration?.

Which compiler you use makes a difference here clang provides the following warning:

warning: empty parentheses interpreted as a function declaration [-Wvexing-parse]
    double moon_g();
                 ^~

while I can not get gcc nor Visual Studio to warn me about this. It is useful in the long run to try code in different C++ compilers when you can, it can be a very educational experience and you don't have to install them either since there are plenty of online C++ compilers available online.