函数重载优先级(参考) [英] Function overloads priority (references)

问题描述

void func(const int &) { std::cout << "c lv ref\n"; }
void func(int &&) { std::cout << "rv ref\n"; }

func(1);

由于const左值引用能够接受各种数据(const和non-lval，const和非右值)，所以我想知道上面的代码将打印"rv ref"的保证是什么.是标准化的还是依赖编译器的?

Since const lvalue reference is able to accept every kind of data (const and non- lval, const and non- rvalue), I wonder what is the guarantee that the code above will print "rv ref". Is it standardized or compiler dependent?

推荐答案

我先前的解释不正确-如

My previous explanation was incorrect - as T.C. mentioned in the comments, both overloads have the same implicit conversion sequence, and the disambiguation happens as a special tie-breaker defined in the standard.

§13.3.3.1.4

当引用类型的参数直接绑定([dcl.init.ref])到参数表达式时，隐式转换序列是身份转换，除非参数表达式具有类型，它是参数类型的派生类，在这种情况下，隐式转换序列是派生到基数转换([over.best.ics]).[...]

When a parameter of reference type binds directly ([dcl.init.ref]) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion ([over.best.ics]). [...]

如果参数类型相同，则应用于两个重载的隐式转换序列是身份转换.这尚未消除这两个功能的歧义.

If the argument types are the same, the implicit conversion sequence applied to both overloads is the identity conversion. This does not yet disambiguate the two functions.

在这里将消歧指定为抢七游戏:

The disambiguation is specified as a tie-breaker here:

§13.3.3.2 [over.ics.rank] ，参数3.2.3

[如果 ...，标准转换序列 S1比标准转换序列S2更好，则是>] S1和S2是参考绑定([dcl.init.ref])都没有引用未声明没有ref限定符的非静态成员函数的隐式对象参数，，并且S1将右值引用绑定到右值，而S2则将左值引用绑定.[...]

[Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if...] S1 and S2 are reference binding ([dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference. [...]

引号表示，对于两个身份隐式转换序列:

• 将右值绑定到右值引用的序列是最好的.

• The sequence binding an rvalue to rvalue reference is the best one.

将右值绑定到左值引用的序列比上述序列差.

The sequence binding an rvalue to an lvalue reference is worse than the aforementioned one.

[...]，如果S1和S2是引用绑定[...]

需要 const 来将右值绑定到 const& 的原因可以在这里找到:

The reason why const is required to bind rvalues to const& can be found here:

§8.6.3 [dcl.init.ref] ，参数5.2

对类型"cv1 T1"的引用由类型"cv2 T2"的表达式初始化，如下所示:

[如果引用是左值引用和初始值设定项表达式...]

否则，引用应为对非易失性const类型的左值引用(即cv1为const)，或者引用应为右值引用.>

Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be const), or the reference shall be an rvalue reference.

如果初始化器表达式 [...] 是右值(但不是位字段)或函数左值且引用了"cv1 T1"-与"cv2 T2"兼容

If the initializer expression [...] is an rvalue (but not a bit-field) or function lvalue and "cv1 T1" is reference-compatible with "cv2 T2"

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