在下面的代码段中不允许在C ++标准中使用`a = b + {1，2}？ [英] Where in the C++ Standard is `a = b + {1, 2}` disallowed in the snippet below?

问题描述

标准中 a = b + {1，2} 以下不允许？

  class complex {
 double re，im; 
 public：
 complex（double r，double i）：re {r}，im {i} {} 
 complex& operator + =（const complex& other）{re + = other.re; im + = other.im; return * this; } 
}; 
 
 inline复数运算符+（复数lhs，const复数和rhs）
 {
 lhs + = rhs; 
 return lhs; 
} 
 
 int main（）
 {
 complex a {1,1} 
 complex b {2，-3}; 
 a + = {1,3}; // Ok 
 a = b + {1,2}; //不编译
} 
  

解决方案

它不被列在N3797§8.5.4[dcl.init.list] / 1（强调我的）：

：列表初始化
可以用作变量定义（8.5）中的初始化器

$ b

• 在返回语句（6.6.3）


• 中的新表达式（5.3.4）


• 中作为初始值的 （5.2.1）


• 作为函数参数（5.2.2）

<

• 作为非静态数据成员（9.2）的初始化器


• 作为参数传递给构造函数调用（8.5,5.2.3）
$

强调的项目符号对应于 a + = {1,3; / code>。没有符合加法参数的点。

Where in the Standard is a = b + {1, 2} disallowed below?

class complex {
    double re, im;
public:
    complex(double r, double i) : re{ r }, im{ i } {}
    complex& operator+=(const complex& other) { re += other.re; im += other.im; return *this; }
};

inline complex operator+(complex lhs, const complex& rhs)
{
    lhs += rhs;
    return lhs;
}

int main()
{
    complex a{ 1, 1 };
    complex b{ 2, -3 };
    a += {1, 3};          // Ok
    a = b + {1, 2};       // doesn't compile
}

解决方案

It is disallowed by not being listed in N3797 §8.5.4 [dcl.init.list]/1 (emphasis mine):

Note: List-initialization can be used

• as the initializer in a variable definition (8.5)
• as the initializer in a new expression (5.3.4)
• in a return statement (6.6.3)
• as a for-range-initializer (6.5)
• as a function argument (5.2.2)
• as a subscript (5.2.1)
• as an argument to a constructor invocation (8.5, 5.2.3)
• as an initializer for a non-static data member (9.2)
• in a mem-initializer (12.6.2)
• on the right-hand side of an assignment (5.17)

The emphasized bullet point corresponds to your a += {1, 3};. There is no point that fits an addition argument.

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