c++ function name ambiguity

Question

My class has two functions with the same name and the following signature:

Mat<T, rows, cols> & transpose()
{   
    for (size_t i = 0; i < rows; ++i) {
        for (size_t j = i + 1; j < cols; ++j) {
            std::swap(at(i, j), at(j, i));
        }
    }
    return *this;
}

This method does an inplace transpose of the matrix. Now, I have another function which leaves the original matrix unchanged and does the transpose in a new matrix. The signature is:

Mat<T, cols, rows> transpose() const

Note that the columns and rows are swapped.

Now, I call it as:

Mat<int, 3, 4> d;
// Fill this matrix
Mat<int, 4, 3> e = d.transpose();

This still tries to call the first method. If I rename the second method to transpose2 and call that, it is fine. Is there a way to make these two functions inambiguous?

Back then when I was reinventing the wheel to learn C++ (like you're doing apparently), I made one of them static and the other a method; or alternatively, I called one `getTranspose()` and the other `transpose()`. — The Quantum Physicist, Aug 15 '17 at 13:53
@LogicStuff Sure I can do that but want to know what I am doing wrong... — Luca, Aug 15 '17 at 13:53
@Luca What you're doing wrong: You can't overload with return value. — The Quantum Physicist, Aug 15 '17 at 13:54
@Someprogrammerdude If I make it const than I cannot fill the matrix...or call any non-const methods. — Luca, Aug 15 '17 at 13:55
@Luca There is no reason the `const` version should be preferred. Nor there is a way, which does not include the instance being `const` or casted to a `const`, or casting a member function pointer to the right overload. — LogicStuff, Aug 15 '17 at 13:56
Thanks for all the comments. I will just rename and get it done with! — Luca, Aug 15 '17 at 13:57
*"This method does an inplace transpose"* - so I would call it `inplace_transpose`. :-) The problem is two functions doing different things, but with the same name. — Bo Persson, Aug 15 '17 at 14:03

songyuanyao · Accepted Answer · 2017-08-16T12:29:57.873

7

The overload resolution doesn't depend on the return value, it's determined by that which function whose parameters match the arguments most closely. You have to make the object being called on to be const, to make the const member function overload to be called. e.g.

Mat<int, 4, 3> e = const_cast<const Mat<int, 3, 4>&>(d).transpose();

edited Aug 16 '17 at 12:29

answered Aug 15 '17 at 13:58

songyuanyao

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@LogicStuff Yes. But shouldn't `const_cast` be more apporpriate here? Because it's relevant to `const`. – songyuanyao Aug 15 '17 at 14:07
It's discussed [here](https://stackoverflow.com/questions/20956757/using-const-cast-to-add-const-ness-bad-idea). It's up to the programmer, however. Some might be used to seeing `const_cast` only for removing cv-qualifiers, some not. – LogicStuff Aug 15 '17 at 14:10
@LogicStuff I can understand that. Honestly I'm not sure myself either. – songyuanyao Aug 15 '17 at 14:12

Geek · Answer 2 · 2017-08-15T13:59:35.477

4

You can't overload function by return type.Discussed in below thread

Is it possible to have different return types for a overloaded method?

In your case, you have overloded on the basis of "const". So second function will be called if you declare d as const otherwise the first version will be called.

edited Aug 15 '17 at 13:59

answered Aug 15 '17 at 13:56

Geek

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c++ function name ambiguity

2 Answers2