C ++中用户定义的转换顺序

Question

I'm facing a problem with a sequence of user-defined conversions in C++. Consider the following code:

struct A
{
    A(int);
};

struct B
{
    B(A);
};

struct C
{
    C(B);
};

The constructors are defined somewhere. Now, the following statements work fine:

A(1);       // A <- int

B(1);       // B <- A <- int

C(A(1));    // C <- B <- A <- int
C(B(1));    // C <- B <- A <- int
C(B(A(1))); // C <- B <- A <- int

However, when I try to compile the following:

C(1);       // C <- B <- A <- int

I get the following error:

error: no matching function for call to ‘C::C(int)’
no known conversion for argument 1 from ‘int’ to ‘B’

I know that such a construction is illegal in C++. Nevertheless, my question is:

• How can I get around it? I'm looking for some elegant solution. The constructions as C(B(A(1))) are somewhat annoying.

---

For completeness, I found several related questions (eg, Why user-defined conversions are limited? ). However, none address my question.

Answer 1

As you've said, your first question is already answered.

As for a work around, assuming you don't want to add an A constructor to your C class, you can just bank on the one allowed implicit conversion:

C(B(1));

The following should also works:

C(A(1));