C++ compilation error: cannot convert from B to A, no constructor, or constructor overload ambiguity

Question

I have some code which implies a type conversion, which does not compile although there is a conversion method...

class A
{
public:
    A(void) :_m(0) { }
    A(int val) : _m(val) {}
private:
    int _m;
};
class B
{
public:
    B(void) : _m(0) {}
    B(int val) : _m(val) {}
    B(const A&);
    // there is a direct conversion operator here
    operator A(void) const { return A(_m); }
    operator int(void) const { return _m; }
private:
    int _m;
};
int main()
{
    B b;
    A a = (A)b; // error C2440 here
}

Here is the error message:

error C2440: 'type cast': cannot convert from 'B' to 'A'
message : No constructor could take the source type, or constructor overload resolution was ambiguous

It took me a while to strip down our actual code, and get this failing code excerpt. Which in itself gave me some insights on what was actually not working... — pascal, Oct 16 '19 at 13:46
remove the c style cast, its unnecessary and seems to be the cause of the issue — Alan Birtles, Oct 16 '19 at 13:49
I tried several ways of writing it, in the actual sources: the given `A a = (A)b`, or 'A a = b`, or 'A a = A(b)`... which all gave error messages, although different ones... They all relate to the excessive possible conversions... — pascal, Oct 16 '19 at 13:52
removing the cast fixes the issue: https://godbolt.org/z/QLJnPv — Alan Birtles, Oct 16 '19 at 13:54
I just made the effort to ask this self-answering question, because I could not find (around C2440) a similar question on the Internet... But i still do not really understand why the compiler did not prefer the conversion operator... — pascal, Oct 16 '19 at 13:54
@pascal Because it's a C style cast. They don't follow the normal C++ casting rules, `static_cast(B)` should work fine. — Mgetz, Oct 16 '19 at 14:16
I've just tried `static_cast(b)`. It did not fix the compilation... — pascal, Oct 16 '19 at 14:36

pascal · Answer 1 · 2019-10-16T13:55:46.013

2

From what I understand, the compiler tries several paths to interpret a = (A)b.

it finds the operator A
but it also finds the operator int on B, and the A(int) constructor which gives it a second path B => int => A...

And it does not know which to pick.

To fix the compilation, I can:

remove the operator int from B
rewrite the error line as A a = b.operator A();...

edited Oct 16 '19 at 13:55

answered Oct 16 '19 at 13:50

pascal

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Vlad from Moscow · Accepted Answer · 2019-10-16T14:12:40.517

The error message means that these two operators

operator A(void) const { return A(_m); }
operator int(void) const { return _m; }

can be used in the expression

(A)b;

As a result using these conversion operators there can be used either the constructor A( int ) or the default copy constructor A( const A & ).

To make it more clear rewrite the corresponding declaration like

A a = A( b );

So whether the object b is converted to an object of the type A using the first conversion operator or to an object of the type int using the second conversion operator.

You could avoid the ambiguity declaring the operators for example like

operator A(void) const & { return A(_m); }
operator int(void) const && { return _m; }

that is for lvalues the first operator will be used and for rvalues the second operator will be used.

Here is your program with the modified operators.

#include <iostream>
class A
{
public:
    A(void) :_m(0) { }
    A(int val) : _m(val) {}
private:
    int _m;
};
class B
{
public:
    B(void) : _m(0) {}
    B(int val) : _m(val) {}
    B(const A&);
    // there is a direct conversion operator here
    operator A(void) const & { return A(_m); }
    operator int(void) const && { return _m; }
private:
    int _m;
};

int main()
{
    B b;
    A a = b; 
    A a1 = B();
}

It seems interesting, although it exceeds my knowledge of C++... but it works. I cannot seem to find a reference docs which shows the "&", "&&" in the conversion function definition.. (granted, the ones I found at Microsoft, IBM, do not seem to allow for "const" either...) — pascal, Oct 16 '19 at 15:02
@pascal A member function can be declared like D1 ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt trailing-return-type — Vlad from Moscow, Oct 16 '19 at 15:08

C++ compilation error: cannot convert from B to A, no constructor, or constructor overload ambiguity

2 Answers2