Function of a function does not recognize an input parameter

Question

I am trying to make a function of a function using pointers to functions, similar to what is given at the bottom (last section) of this link on www.cplusplus.com, except a little more advanced. I am trying the following:

In myFile.h

// namespace for: Functions
namespace Functions {

    // namespace for: 1D functions
    namespace OneDimensional {

        // Function for: f(x) = x * x, Note: read t as times
        double xtx(double x);
    }

    // namespace for: 2D functions
    namespace TwoDimensional {

    // Function for: f(x, g(y)) = x + g(y), Note: read _ as "of"
    double f_xANDg_y(double x, double(*g)(double y));
    }
}

In myFile.cpp

double Functions::OneDimensional::xtx(double x) {
    return (x * x);
}

double Functions::TwoDimensional::f_xANDg_y(double x, double(*g)(double y)) {
    return (x + (*g)(y));  // <== This is where I get the Error (E0020)
}

I checked the error E0020 and this brought me to Stack Overflow and that user was missing a brace. I checked and I have no missing brace (but I could be wrong even after checking a few times).

Is it that I am implementing this idea of f(x, g(y)) incorrectly, or am I actually missing a brace?

What's y? Which value do you want to pass to g? Did you mean to pass y as an extra parameter? It's difficult to tell what you are trying to achieve, but perhaps something like double f_xANDg_y(double x, double y, double(*g)(double)) { return x + g(y); } I don't see the point of the exercise though. — Igor Tandetnik
– Igor Tandetnik, Commented Aug 4, 2018 at 23:47
@IgorTandetnik My idea is to pass g which is a pointer to some function (any function) which itself has a input parameter y. That is what I am trying to implement in myFile.cpp — G. LC
– G. LC, Commented Aug 4, 2018 at 23:50
Well, you did pass the function. Now you also need to pass a value this function should be called with. — Igor Tandetnik
– Igor Tandetnik, Commented Aug 4, 2018 at 23:50
@IgorTandetnik Ok, I will try with the way you said of trying with 3 input parameters to f_xAND_y, thanks for the help — G. LC
– G. LC, Commented Aug 4, 2018 at 23:52
@IgorTandetnik That took care of the error, if you want to post it as an answer you can, either way thanks also the point of this exercise is that I am need these functions for testing numerical methods that I am writing as well — G. LC
– G. LC, Commented Aug 4, 2018 at 23:54

Mohammed Zaed · Accepted Answer · 2018-08-05 15:57:47Z

The error appears here because the compile doesn't know what is "y", since it is not declared any where in the visible scope of the function call. 'f_xANDg_y(double x, double(*g)(double y);' The above function parameter takes only two parameters,

"x" - which 'is of type double'
"g" as pointer to function of type 'double(*)(double)'. which is 'double (*g) (double y)' here 'y' is part of function pointer, your code would have written function perfectly with out using the y.
Here you are trying call your function with a 'y' as input parameter and compiler is not able to find 'double y' in its scope, hence, it throws an error.
Since the whole 'double(*g)(double y)' argument in your function TwoDimenesional boil downs to "a function pointer which take double as input and returns double"
```
    double Functions::TwoDimensional::f_xANDg_y(double x, double(*g)(double y)) {
   return (x + (*g)(y));  // <== This is where I get the Error (E0020)
}
```
Your code works fine, you will need to add one more "double" as an argument to your function as shown here TwoDimensional::f_xANDg_y(double x, double(*g)(double), double y). As expected by the compiler.
Your code after making the corrections:

'

namespace Functions {

    // namespace for: 1D functions
    namespace OneDimensional {

        // Function for: f(x) = x * x, Note: read t as times
        double xtx(double x);
    }

    // namespace for: 2D functions
    namespace TwoDimensional {

    // Function for: f(x, g(y)) = x + g(y),a Note: read _ as "of"
    double f_xANDg_y(double x, double(*g)(double), double yParam);
    }
}

double Functions::OneDimensional::xtx(double x) {
    return (x * x);
}

double Functions::TwoDimensional::f_xANDg_y(double x, double(*g)(double), double y) {
    return (x + g(y));
}

'

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