支持的初始化器列表中是否允许显式转换操作符? [英] Are explicit conversion operators allowed in braced initializer lists?
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问题描述
以下代码使用GCC 4.9.2编译,但不与Clang 3.5.0一起编译:
#include< string&
类Foo
{
public:
显式运算符std :: string()const;
};
std :: string bar {Foo {}}; //在g ++中工作,在clang中失败++
std :: string baz(Foo {}); // works in both
clang ++说:
foo.cpp:9:13:错误:没有匹配的构造函数初始化'std :: string'
(aka'basic_string< char>')
std :: string bar {Foo {}};
^ ~~~~~~~
...:注意:候选构造函数不可行:没有已知的从'Foo'到
的转换const std :: basic_string< char> &'for 1st argument
basic_string(const basic_string& __str);奇怪的是,如果 std :: string / code>替换为 int
之类的原始类型。解决方案这似乎是一个ang错误。 [over.match.list] / 1:
当非聚合类类型 T
是列表初始化
(8.5.4),重载解决方案在两个阶段选择构造函数:
- ..]
- 如果没有找到可行的初始化列表构造函数,则再次执行重载解析,其中候选函数都是
类的构造函数<$
/ blockquote>
由于第二行编译正确,因此存在不一致:当涉及重载解析时,它们应该是等效的。
The following code compiles with GCC 4.9.2 but not with Clang 3.5.0:
#include <string>
class Foo
{
public:
explicit operator std::string() const;
};
std::string bar{Foo{}}; // Works in g++, fails in clang++
std::string baz(Foo{}); // Works in both
clang++ says:
foo.cpp:9:13: error: no matching constructor for initialization of 'std::string'
(aka 'basic_string<char>')
std::string bar{Foo{}};
^ ~~~~~~~
...: note: candidate constructor not viable: no known conversion from 'Foo' to
'const std::basic_string<char> &' for 1st argument
basic_string(const basic_string& __str);
^
Curiously, it works if std::string
is replaced with a primitive type like int
.
解决方案 This seems to be a Clang bug. [over.match.list]/1:
When objects of non-aggregate class type T
are list-initialized
(8.5.4), overload resolution selects the constructor in two phases:
- [..]
- If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all
the constructors of the class
T
and the argument list consists of
the elements of the initializer list.
Since the second line compiles fine, there is an inconsistency: They should be equivalent when it comes to overload resolution.
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