Implicit conversions in C++

Question

Given the following code, why doesn't the compiler resolve the implicit conversion when constructing Bar? That is, construct Foo just like a was constructed which is (should) then be used to construct Bar?

#include <string>

class ImplicitlyConvertToChar
{
public:
  ImplicitlyConvertToChar(const char* a_char)
    : m_string(a_char)
  { }

  ImplicitlyConvertToChar(const char* a_char, size_t a_end)
    : m_string(a_char)
  {
  }

  template <typename T_String>
  ImplicitlyConvertToChar(T_String const& a_string)
    : m_string(a_string.begin())
  {
  }

  operator char const * () const
  { return m_string; }

  const char* m_string;
};

class Foo
{
public:

  Foo(const ImplicitlyConvertToChar& a_charLike)
    : m_string(a_charLike)
  { }

  const char* m_string;
};

class Bar
{
public:
  Bar(const Foo& a_foo)
    : m_foo(a_foo)
  { }

  Foo m_foo;
};

int main()
{
  Foo a("this works");
  Bar b("Why doesn't this?");
}

It's part of the standard, implicit conversions can't be chained like that. You can only have one implicit conversion. — IronMensan
– IronMensan, Commented Aug 12, 2013 at 15:23

juanchopanza · Accepted Answer · 2013-08-12 15:23:39Z

9

You are not allowed more than one user defined implicit conversion. The Foo example involves one, the Bar example involves two.

answered Aug 12, 2013 at 15:23

juanchopanza

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Comments

zmb · Accepted Answer · 2013-08-12 15:46:41Z

4

The compiler is only allowed to make a single implicit user-defined conversion.

See http://en.cppreference.com/w/cpp/language/implicit_cast

A user-defined conversion consists of: 
    zero or one non-explicit single-argument constructor or non-explicit 
    conversion function calls

Constructing Bar that way would require two.

edited Aug 12, 2013 at 15:46

answered Aug 12, 2013 at 15:25

zmb

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4 Comments

Armen Tsirunyan Over a year ago

Your opening sentence is not correct. The compiler is allowed to make a single implicit user-defined conversion. It can perform as many implicit standard conversion as it likes

Martin James Over a year ago

I wish it liked none.

zmb Over a year ago

@ArmenTsirunyan thanks for pointing out this critical oversight. Answer updated.

Trent Gm Over a year ago

@ArmenTsirunyan, en.cppreference.com/w/cpp/language/implicit_cast states that only up to two standard conversions can be performed

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