While looking at this question I tried it out with clang and got in a weird situation. The following example:
#include <string>
class ACP
{
public:
ACP() {}
operator const std::string() const { return std::string(); }
// operator std::string() const { return std::string(); } <-- makes clang happy
};
void test()
{
const ACP acp;
auto a = (std::string)acp;
}
compiles fine on coliru with gcc, but fails with clang. At least I see no issue with this example - is this a bug in clang or is there a rule that actually explains the clang error and gcc is wrong?
The error from clang can be seen below:
clang -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
main.cpp:13:26: error: no viable conversion from 'const ACP' to 'std::__cxx11::basic_string<char>'
auto a = (std::string)acp;
^~~
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../include/c++/7.1.0/bits/basic_string.h:421:7: note: candidate constructor not viable: no known conversion from 'const ACP' to 'const std::__cxx11::basic_string<char> &' for 1st argument
basic_string(const basic_string& __str)
^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../include/c++/7.1.0/bits/basic_string.h:493:7: note: candidate constructor not viable: no known conversion from 'const ACP' to 'const char *' for 1st argument
basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../include/c++/7.1.0/bits/basic_string.h:515:7: note: candidate constructor not viable: no known conversion from 'const ACP' to 'std::__cxx11::basic_string<char> &&' for 1st argument
basic_string(basic_string&& __str) noexcept
^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../include/c++/7.1.0/bits/basic_string.h:542:7: note: candidate constructor not viable: no known conversion from 'const ACP' to 'initializer_list<char>' for 1st argument
basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
^
main.cpp:7:5: note: candidate function
operator const std::string() const { return std::string(); }
^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../include/c++/7.1.0/bits/basic_string.h:515:35: note: passing argument to parameter '__str' here
basic_string(basic_string&& __str) noexcept
^
1 error generated.
However I can't see why the compiler could not use the copy ctor from std::string.