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Editorial Team
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Asked: 2026-05-23T07:59:03+00:00

Consider the following C++ code: struct B { }; struct A { A(int); A(A&);

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Consider the following C++ code:

struct B { };
struct A
{
        A(int);
        A(A&); // missing const is intentional
        A(B);
        operator B();
};

A f()
{
        // return A(1); // compiles fine
        return 1; // doesn't compile
}

This compiles fine on MSVC++ 2010 (in fact, on MSVC it even works if I remove B altogether). It doesn’t on GCC 4.6.0:

conv.cpp: In function ‘A f()’:
conv.cpp:13:9: error: no matching function for call to ‘A::A(A)’
conv.cpp:13:9: note: candidates are:
conv.cpp:6:2: note: A::A(B)
conv.cpp:6:2: note:   no known conversion for argument 1 from ‘A’ to ‘B’
conv.cpp:5:2: note: A::A(A&)
conv.cpp:5:2: note:   no known conversion for argument 1 from ‘A’ to ‘A&’
conv.cpp:4:2: note: A::A(int)
conv.cpp:4:2: note:   no known conversion for argument 1 from ‘A’ to ‘int’

What’s confusing me is the message no known conversion for argument 1 from ‘A’ to ‘B’. How can this be true considering that A::operator B() is very well defined?

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1 Answer

  1. Editorial Team

    The error is quite clear on the list of candidates that were rejected. The problem is that implicit conversion sequences involving a user defined conversion in the C++ language are limited to a single user defined conversion:

    §13.3.3.1.2 [over.ics.user]/1 A user-defined conversion sequence consists of an initial standard conversion sequence followed by a user-defined conversion (12.3) followed by a second standard conversion sequence.

    The standard conversion sequences are defined in §4[conv]:

    […] A standard conversion sequence is a sequence of standard conversions in the following order

    • Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.

    • Zero or one conversion from the following set: integral promotions, floating point promotion, integral conversions, floating point conversions, floating-integral conversions, pointer conversions, pointer to member conversions, and boolean conversions.

    • Zero or one qualification conversion.

    The problem is that your code cannot get from point a) int rvalue to point b) B by applying a single user defined conversion.

    In particular, all conversion sequences that are available start with a user defined conversion (implicit constructor A(int)) that yield an A rvalue. From there, the rvalue cannot be bound to a non-const reference to call A::A( A& ), so that path is discarded. All the other paths require a second user defined conversion that is not allowed, and in fact the only other path that would get us to point b) requires two other user defined conversions for a total of 3.

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