Calling a function in main

问题:

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 any mass on the moon\n";

    double moon_g();

}

double moon_g (double a, double b)
{
    cout<<"Enter the mass in kilograms. Use decimal point for any number entered";
    cin>>a;
    b=(17*9.8)/100;
    double mg=a*b;
    return mg;
}

It compiles, but when I run it it only prints out:

This program will calculate the weight of any mass on the moon

but doesn't execute the moon_g function.

回答1:

This line:

double moon_g();

doesn't actually do anything, it just states that a function double moon_g() exists. What you want is something like this:

double weight = moon_g();
cout << "Weight is " << weight << endl;

This won't work yet, because you don't have a function double moon_g(), what you have is a function double moon_g(double a, double b). But those arguments aren't really used for anything (well, they are, but there's no reason to have them passed in as arguments). So eliminate them from your function like so:

double moon_g()
{
  cout<<"Enter the mass in kilograms. Use decimal point for any number entered";
  double a;
  cin>>a;
  double b=(17*9.8)/100;
  double mg=a*b;
  return mg;
}

(And declare the function before you call it.) More refinements are possible, but that'll be enough for now.

回答2:

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.

回答3:

There is huge difference between calling a function and declaring it just as there is difference between local variables and function arguments.

I suggest reading basic tutorials first.

Anyway, thats how code should look like:

#include <iostream>
using namespace std;

double moon_g ()
{
    double a,b;
    cout<<"Enter the mass in kilograms. Use decimal point for any number entered\n";
    cin>>a;
    b=(17*9.8)/100;
    double mg=a*b;
    return mg;
}

int main()
{
    cout<<"This program will calculate the weight of any mass on the moon\n";

    cout<<"Result is: "<<moon_g();
}

回答4:

There are two problems in your code.

Firstly, if you want to call your function

double moon_g (double a, double b) // this means if you want to call moon_g() you must provide arguments a and b, otherwise, the you will encounter an compile error.
{
    cout<<"Enter the mass in kilograms. Use decimal point for any number entered";
    cin>>a;
    b=(17*9.8)/100;
    double mg=a*b;
    return mg;
}

you should provide the two parameters a and b. But a and b are calculated in the body of function definition, it is unnecessary to declare the two parameters. You can write like this.

double moon_g () //this means function moon_g() does not accept any arguments
{
    double a, b; // declare a and b in the definition body instead of in the arguments list
    cout<<"Enter the mass in kilograms. Use decimal point for any number entered";
    cin>>a;
    b=(17*9.8)/100;
    double mg=a*b;
    return mg;
}

Then, in the main function, your calling function statement is wrong. You may want to receive the return value. So, you should write the code like this.

int main()
{
    cout<<"This program will calculate the weight of any mass on the moon\n";

    double ret = moon_g();
}

Finally, it is mostly recommended that the function which will be called by another function should be declared or defined previously.