问题

以下是您要翻译的内容：

原始代码：

using namespace std::placeholders;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

代码运行正常，但 cpplint 报告说我应该使用 "using-declarations"，然后我将代码更改为：

using _1 = std::placeholders::_1;
using _2 = std::placeholders::_2;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

但现在编译器报错：

error: ‘_1’ in namespace ‘std::placeholders’ does not name a type

应该使用正确的 using-declarations 吗？

更新
到目前为止的回答建议我重新添加这行：

using namespace std::placeholders;

正如我所描述的，cpplint 不喜欢使用命名空间，这是 cpplint 的消息：

Do not use namespace using-directives. Use using-declarations instead.

您能建议如何处理这个问题吗？

using namespace std::placeholders;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

using _1 = std::placeholders::_1;
using _2 = std::placeholders::_2;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

error: ‘_1’ in namespace ‘std::placeholders’ does not name a type

Do not use namespace using-directives.  Use using-declarations instead.

答案1

得分: 1

std::placeholders::_1 不是一种类型，而是一个未指定类型的对象，因此不能对其应用using指令。只需将namespace std::placeholders引入所需的作用域即可。

using namespace std::placeholders;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

using namespace std::placeholders;
my_cb = std::bind(&Myclass::handler, this, _1, _2);

答案2

得分: 1

以下是翻译好的部分：

"The message wants you to write" -> "这条消息要求你编写"
"using std::placeholders::_1;" -> "使用 std::placeholders::1;"
"using std::placeholders::2;" -> "使用 std::placeholders::2;"
"/.../" -> "/.../"
"etc. This is what using declarations are." -> "等等。这就是using声明的作用。"
"What you tried with syntax of the form using /*...*/ = /*...*/; are type aliases," -> "你尝试的形式为using /*...*/ = /*...*/;是_类型别名，"
"a third meaning of using distinct from both using directives (using namespace /*...*/; and using declarations (using /*...*/;)."
-> "这是using的第三种含义，与_使用指令（using namespace /*...*/;）和_使用声明（using /*...*/;）都不同。"
"However, I don't think that is technically allowed at global namespace scope because names starting with an underscore are reserved there." -> "但是，我认为从技术上讲，在全局命名空间范围内是不允许的，因为以下划线开头的名称在那里是保留的。"
"Do it in a narrower scope." -> "在更窄的范围内执行它。"
"The message is from clang-tidy and implements a particular rule in Google's style guides that flat out forbids any use of using namespace." -> "这条消息来自clang-tidy，并实施了Google风格指南中明令禁止使用using namespace的特定规则。"
"However, the std::placeholders namespace is specifically designed to be used in this way and avoids making any other names visible." -> "然而，std::placeholders命名空间是专门设计用于这种方式，并避免使其他名称可见。"
"As long as it is used in a narrow scope (not at the global scope) I would consider it fine." -> "只要它在较窄的范围内使用（而不是在全局范围内），我认为它是可以接受的。"
"Opinions on this rule will differ." -> "关于这个规则的意见会有不同。"

using std::placeholders::_1;
using std::placeholders::_2;
/*...*/