下标迭代器中的lambda [英] lambda inside subscript iterator

问题描述

在下标运算符中包含lambda似乎不适用于g ++和clang.

Having a lambda in a subscript operator seems to be not working for g++ and clang.

这是实现错误还是c ++标准中的不满意"规则?

Is this an implementation error or a "unhappy" rule in c++ standard?

示例:

class A
{   
    public:
        template<typename T> void operator[](T) {}
        template<typename T> void operator()(T) {}
};  

int main()
{   
    A a;
    a[ [](){} ];    // did not compiler: see error message
    a( [](){} );    // works as expected
} 

错误:

main.cpp:13:6: error: two consecutive '[' shall only introduce an attribute before '[' token
     a[ [](){} ];
      ^   
main.cpp:13:15: error: expected primary-expression before ']' token
     a[ [](){} ];

我知道属性以"[["开头，但是我想知道"[["(带有一个或多个空格)也可以像这样工作:

I know that attributes starts with "[[" but I am wondering that "[ [" ( with one or more white spaces ) also works like:

 void func( int x [ [gnu::unused] ] ) {} // compiles fine! :-(

推荐答案

[dcl.attr.grammar]中对此进行了介绍.具有两个连续的[是属性，因此您必须将其包装在括号中或执行其他操作以使您的意图清楚:

This is covered in [dcl.attr.grammar]. Having two consecutive [ is an attribute, so you'll have to wrap in parantheses or do something else to make your intent clear:

在引入属性说明符时或在其内部时，两个连续的左方括号标记应仅出现 属性自变量子句的 balanced-token-seq . [注意:如果出现两个连续的左方括号 如果不允许使用属性说明符，则该程序格式不正确，即使括号与替代语法产生符匹配也是如此. -尾注] [示例:

Two consecutive left square bracket tokens shall appear only when introducing an attribute-specifier or within the balanced-token-seq of an attribute-argument-clause. [ Note: If two consecutive left square brackets appear where an attribute-specifier is not allowed, the program is ill-formed even if the brackets match an alternative grammar production. —end note ] [ Example:

int p[10];
void f() {
  int x = 42, y[5];
  int(p[[x] { return x; }()]);   // error: invalid attribute on a nested
                                 // declarator-id and not a function-style cast of
                                 // an element of p.
  y[[] { return 2; }()] = 2;     // error even though attributes are not allowed
                                 // in this context.
  int i [[vendor::attr([[]])]];  // well-formed implementation-defined attribute.
}

-结束示例]

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