为什么我可以在运算符+ =的右侧使用初始化程序列表，但不能在运算符+的右侧使用初始化程序列表? [英] Why can I use initializer lists on the right-hand side of operator += but not operator+?

问题描述

这是对这个较早的问题.

请考虑以下C ++代码，您可以在ideone.com上直播

Consider the following C++ code, which you can try live at ideone.com:

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

struct AddInitializerList {
    void operator+= (initializer_list<int> values) {
        // Do nothing   
    }

    void operator+ (initializer_list<int> values) {
        // Do nothing
    }
};

int main() {
    AddInitializerList adder;
    adder += {1, 2, 3};  // Totally legit
    adder +  {1, 2, 3};  // Not okay!

    return 0;
}

main中将operator+与大括号括起来的初始化程序列表一起使用的行不会编译(并且，在问了前面的问题之后，我现在知道这是为什么).但是，我很困惑为什么在main中使用opeartor+=的代码确实可以很好地编译.

The line in main that uses operator+ with a brace-enclosed initializer list does not compile (and, after asking that earlier question, I now know why this is). However, I'm confused why the code that uses opeartor+= in main does indeed compile just fine.

我对为什么可以重载+=并使它正常工作感到困惑，而重载+似乎在这里不起作用.标准中是否有特定规定允许在+=运算符而不是+运算符的上下文中使用大括号括起来的初始化程序?还是这只是一个奇怪的编译器怪癖?

I'm confused as to precisely why I can overload += and have it work just fine, while overloading + doesn't seem to work here. Is there a particular provision in the standard that permits brace-enclosed initializers in the context of the += operator but not the + operator? Or is this just a weird compiler quirk?

推荐答案

It is explained in the answer to this question (which is linked from the question you linked to).

语言语法仅在特定的语法环境中允许使用括号列表，而不是任意表达式.该列表包括赋值运算符的右侧，但通常不包括运算符的右侧.

The language grammar only allows a braced list in certain grammatical contexts, not in place of an arbitrary expression. That list includes the right-hand side of assignment operators, but NOT the right-hand side of operators in general.

+=是赋值运算符，+不是.

+= is an assignment operator, + is not.

赋值表达式的语法是:

   assignment-expression:
     条件表达式
     逻辑或表达式分配运算符初始化程序子句
      throw-expression 
   assignment-operator:其中之一
      = *= *= /= %= += -= >>= <<= &= ^= |=
  

  assignment-expression:
     conditional-expression
     logical-or-expression assignment-operator initializer-clause
     throw-expression
  assignment-operator: one of
      = *= *= /= %= += -= >>= <<= &= ^= |=
  

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