是免费的operator-> *重载的邪恶？ [英] Are free operator->* overloads evil?

问题描述

在驳斥观点

I was perusing section 13.5 after refuting the notion that built-in operators do not participate in overload resolution, and noticed that there is no section on operator->*. It is just a generic binary operator.

它的兄弟， operator-> ，运算符* 和 operator [] 都需要是非静态成员函数。这排除了对用于从对象获得引用的操作符 的自由函数重载的定义。但是不包括不常见的 operator-> * 。

Its brethren, operator->, operator*, and operator[], are all required to be non-static member functions. This precludes definition of a free function overload to an operator commonly used to obtain a reference from an object. But the uncommon operator->* is left out.

特别是 [] 有很多相似之处。它是二进制的（他们错过了一个黄金的机会使它成为n元），它接受左边的某种容器和右边的某种定位器。它的特别规则一节，13.5.5，似乎没有任何实际效果，除了非法自由功能。 （这种限制甚至排除了对交换性的支持！）

In particular, operator[] has many similarities. It is binary (they missed a golden opportunity to make it n-ary), and it accepts some kind of container on the left and some kind of locator on the right. Its special-rules section, 13.5.5, doesn't seem to have any actual effect except to outlaw free functions. (And that restriction even precludes support for commutativity!)

因此，例如，这是完美法律：

#include <utility>
#include <iostream>
using namespace std;

template< class T >
T &
operator->*( pair<T,T> &l, bool r )
    { return r? l.second : l.first; }

template< class T >
 T & operator->*( bool l, pair<T,T> &r ) { return r->*l; }

int main() {
        pair<int, int> y( 5, 6 );
        y->*(0) = 7;
        y->*0->*y = 8; // evaluates to 7->*y = y.second
        cerr << y.first << " " << y.second << endl;
}

很容易找到用途，但是替代语法不会那么糟糕。例如，向量的缩放索引：

It's easy to find uses, but alternative syntax tends not to be that bad. For example, scaled indexes for vector:

v->*matrix_width[2][5] = x; // ->* not hopelessly out of place

my_indexer<2> m( v, dim ); // my_indexer being the type of (v->*width)
m[2][5] = x; // it is probably more practical to slice just once

标准委员会忘了防止这种情况，

Did the standards committee forget to prevent this, was it considered too ugly to bother, or are there real-world use cases?

推荐答案

Googling一点，我发现更多的实例的人询问是否使用 operator-> * 比实际建议。

Googling around a bit, I found more instances of people asking whether operator->* is ever used than actual suggestions.

几个地方建议 T& A :: operator-> *（TB :: *）。不确定这是否反映了设计师的意图或者错误地描述 T& A :: operator-> *（T A :: *）是内置函数。不是真的与我的问题有关，而是给出了我在在线讨论&文学。

A couple places suggest T &A::operator->*( T B::* ). Not sure whether this reflects designer's intent or a misimpression that T &A::operator->*( T A::* ) is a builtin. Not really related to my question, but gives an idea of the depth I found in online discussion & literature.

有一个提到D& E 11.5.4，我想是C ++的设计和进化。也许这包含一个提示。否则，我只是得出结论，这是一个无用的丑陋，被标准化和大多数其他人忽略了。

There was a mention of "D&E 11.5.4" which I suppose is Design and Evolution of C++. Perhaps that contains a hint. Otherwise, I'm just gonna conclude it's a bit of useless ugliness that was overlooked by standardization, and most everyone else too.

编辑

为了定量化， - > * 是D&最紧密的绑定操作符，可以通过自由函数重载。所有后缀表达式和一元运算符重载需要非静态成员函数签名。一元运算符之后的优先级是C风格的转型，这可以说是对应于转换函数（ operator type（）），也不能是自由函数。然后是 - > * ，然后乘法。 - > * 可能像 [] 或像％，他们可以采取任何一种方式，他们选择 EEEEEEVIL 的路径。

To put this quantitatively, ->* is the tightest binding operator that can be overloaded by a free function. All the postfix-expression and unary operators overloads require nonstatic member function signatures. Next precedence after unary operators are C-style casts, which could be said to correspond to conversion functions (operator type()), which also cannot be free functions. Then comes ->*, then multiplication. ->* could have been like [] or like %, they could have gone either way, and they chose the path of EEEEEEVIL.

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