Calling method on method in C++

Question

in the method bool Foam::pimpleControl::criteriaSatisfied() of an OpenFoam Source Code I found the following expression:

bool Foam::pimpleControl::criteriaSatisfied()
{
    // ...
    const word& variableName = iter().keyword();
    // ...
}

For iter() I found: Foam::label iter() const inline Return const access to the current cloud iteration

For keyword():

keyType& keyword() inline Return non-const access to keyword.

I have two questions about this:

What does it mean when calling method on method like iter().keyword();?
What does & after word& or keyType& mean? I know that every method also has a datatyp declaration, but has the & a specific meaning?

greetings Streight

"Foam::label iter() const inline Return const access to the current cloud iteration" - Come again? — David G
– David G, Commented Oct 28, 2013 at 18:55

Igor Tandetnik · Accepted Answer · 2013-10-28 18:58:43Z

4

What does it mean when calling method on method like iter().keyword();

iter() returns an object. keyword() then calls a method of that object. It's basically just a shorthand for Foam::label temp = iter(); temp.keyword().

What does & after word& or keyType& mean?

The ampersand is part of the type. const word& names the type "reference to const word".

answered Oct 28, 2013 at 18:58

Igor Tandetnik

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Comments

kfsone · Accepted Answer · 2013-10-28 18:59:50Z

1

You're not calling a "method on method". "iter()" is returning an object and that object has a member "keyword()".

The code is equivalent to

Foam::label& it = iter();
it.keyword();

here's a simplified example:

class A {
    int m_i;
public:
    A(int i) : m_i(i) {}
    int GetI() const { return m_i; }
};

class B {
    A m_a;
public:
    B(int i) : m_a(i) {}
    const A& GetA() const { return m_a; }
};

#include <iostream>

int main() {
    B b(42);

    const A& a = b.GetA();
    std::cout << "a.GetI() == " << a.GetI() <<std::endl;

    // above code is directly equivalent to:
    std::cout << "b.GetA().GetI() == " << b.GetA().GetI() <<std::endl;
}

In the last line, "b.GetA()" returns a temporary, anonymous "const A&" object, and then we call "GetI()" on that object.

answered Oct 28, 2013 at 18:59

kfsone

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1 Comment

Streight Over a year ago

Thx for the example, I just gave the other answer the solved flag, because it appeared first and also answered my question.

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