我有这个代码
struct A { A(); A(A&); };
struct B { B(const A&); };
void f(A);
void f(B);
int main() {
f(A());
}
令我惊讶的是,GCC 和 Clang 失败了。 Clang 举例说
Compilation finished with errors:
source.cpp:8:10: error: no matching constructor for initialization of 'A'
f(A());
^~~
source.cpp:1:21: note: candidate constructor not viable: expects an l-value for 1st argument
struct A { A(); A(A&); };
^
source.cpp:1:16: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
struct A { A(); A(A&); };
^
source.cpp:4:13: note: passing argument to parameter here
void f(A);
为什么他们选择第一个 f
,而第二个 f
工作正常?如果我删除第一个 f
,则调用成功。对我来说更奇怪的是,如果我使用大括号初始化,它也可以正常工作
int main() {
f({A()});
}
他们都叫第二个f
。
最佳答案
这是一种语言怪癖。第一个f
匹配更好,因为您的 A
不需要转换来匹配参数类型( A
),但是当编译器尝试进行调用时,找不到合适的复制构造函数的事实会导致调用失败。在执行重载解决步骤时,该语言不允许考虑实际调用的可行性。
最匹配的标准引用 ISO/IEC 14882:2011 13.3.3.1.2 用户定义的转换序列 [over.ics.user]:
A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a copy/move constructor (i.e., a user-defined conversion function) is called for those cases.
对于列表初始化情况,您可能需要查看: 13.3.3.1.2 用户定义的转换序列[over.ics.user]
When objects of non-aggregate class type T are list-initialized (8.5.4), overload resolution selects the constructor in two phases:
— Initially, the candidate functions are the initializer-list constructors (8.5.4) of the class T and the argument list consists of the initializer list as a single argument.
— 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.
因为重载解决方案必须在每种情况下为 f(A)
查看可行的构造函数和 f(B)
它必须拒绝尝试绑定(bind) A()
的序列至A(A&)
但是 B(const A&)
还是可行的。
关于c++ - 编译器认为 "A(A&)"暂时接受右值?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15852528/