CODEWITHSUNDEEP
c++c++11constructorimplicit-conversioncopy-initialization
fincs
fincs

Reputation: 207

C++ copy initialization + implicit constructor call = fail

This code:

class foo
{
    int x;
public:
    foo(int x) : x(x) { }
    int get() const { return x; }
    //...
};

class bar
{
    int x;
public:
    bar(const foo& x) : x(x.get()) { }
    int get() const { return x; }
    bar& operator =(const foo& rhs) { x = rhs.get(); return *this; }
    //...
};

void func()
{
    foo f = 3;
    bar b = 3;
    b = 7;
    //...
}

errors out on the bar b = 3 line (g++ 4.7.1 with -fstd=gnu++11):

error: conversion from 'int' to non-scalar type 'bar' requested

However, I'm providing a bar constructor that takes a foo, and ints can be implicitly converted to foo as shown by the line preceding it. So, what is going wrong?

By the way, for several reasons it is undesirable to force conversion to foo using foo(3) because that would make my actual code ugly to use and read.

Upvotes: 1

Views: 151

Answers (2)

Andy Prowl
Andy Prowl

Reputation: 126412

§ 13.3.3.1.2/1 about implicit user-defined conversion sequences mandates:

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. If the user-defined conversion is specified by a constructor (12.3.1), the initial standard conversion sequence converts the source type to the type required by the argument of the constructor. If the user-defined conversion is specified by a conversion function (12.3.2), the initial standard conversion sequence converts the source type to the implicit object parameter of the conversion function.

This means it is not possible to chain more than one user-defined conversion.

Upvotes: 0

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361302

I'm providing a bar constructor that takes a foo, and ints can be implicitly converted to foo as shown by the line preceding it. So, what is going wrong?

Chained-conversion is not allowed in C++, which means the (chained) conversion cannot occur:

int -> foo -> bar  //not allowed (chained conversion)

even though the followings are given:

int -> foo  //given
foo -> bar  //given

So if you want int -> bar to work, then add another constructor taking int to class bar.

Upvotes: 5

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