The following code compiles with GCC 4.9.2 but not with Clang 3.5.0:
#include
class Foo
{
public:
explicit operator std::string() const;
};
-
clang doesn't seem to care whether the conversion operator is explicit
or not, and I believe it is correct due to the wording in [over.best.ics].
First of all, the direct-initialization
std::string baz(Foo{});
works on both gcc and clang, and is explained by [class.conv.fct]/2 as mentioned in KerrekSB's answer.
The direct-list-initialization
std::string bar{Foo{}};
on the other hand, does not consider any user defined conversions, explicit
or not.
Quoting N3337, §13.3.3.1/4 [over.best.ics]
However, when considering the argument of a constructor or user-defined conversion function that is a candidate by 13.3.1.3 when invoked for the copying/moving of the temporary in the second step of a class copy-initialization, by 13.3.1.7 when passing the initializer list as a single argument or when the initializer list has exactly one element and a conversion to some class X or reference to (possibly cv-qualified) X is considered for the first parameter of a constructor of X, or by 13.3.1.4, 13.3.1.5, or 13.3.1.6 in all cases, only standard conversion sequences and ellipsis conversion sequences are considered.
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