Question

I have come across a situation that conflicts with my current understanding of methods in C++.

I am working through Ivor Horton’s “Beginning Visual C++ 2010” (Wrox Press). On page 449, Example 8_03, a method is defined as:

double Volume() const {
    return m_Length * m_Width * m_Height;
}

I had rearranged the modifiers as:

double **const** Volume() {
    return m_Length * m_Width * m_Height;
}

From my C# and Java background, I had expected the position of const to be irrelevant, but on compilation I received the error:

error C2662: 'CBox::Volume' : cannot convert 'this' pointer from 
             'const CBox' to 'CBox &'

The error disappears when I return the order to the way Ivor has it.

Does the order in fact make a difference and this not some exotic bug ? If order does matter, how does one remember the correct positions ?

Thanks,

Scott

Answer 1

When const is placed after the name of a member method, that is stating that the this pointer is what is constant. That is to say, the original declaration states that the method CBox::Volume() does not change the CBox object on which it is called.

The most likely source of error is that the CBox::Volume() function is being called on a const CBox, or inside another const method of that CBox.