来自initializer_list的三元运算符+ C ++ 11构造函数 [英] Ternary operator + C++11 constructor from initializer_list

问题描述

在开发应用程序时，我遇到以下问题.当给定的函数指针为null时，我想返回一个空的 std :: list< string> ，否则返回该函数的结果.这是我的代码的简化版本:

  typedef std :: list< std :: string>(* ParamGenerator)();std :: list< std :: string>foo(){/* ... */ParamGenerator generator = ...;如果(发电机)返回generator();别的返回 {};} 

但是，在这些情况下，我通常喜欢使用三元(?:)运算符，因此我尝试以这种方式使用它(照常):

 返回生成器?发电机() : {}; 

但是出现了这个错误:

  somefile.cpp:143:46:错误:"{"令牌之前的预期主表达式somefile.cpp:143:46:错误:在"{"令牌之前应为;" 

这是否意味着我不能使用三元运算符从 initializer_list 返回使用其构造函数创建的对象?有什么特别的原因吗?

解决方案

8.5.4.1中的标准编写:列表初始化

注意:可以使用列表初始化

• 作为变量定义(8.5)中的初始化程序
• 作为新表达式(5.3.4)中的初始化程序
• 在返回声明中(6.6.3)
• 作为函数参数(5.2.2)
• 作为下标(5.2.1)
• 作为构造函数调用的参数(8.5、5.2.3)
• 作为非静态数据成员(9.2)的初始化程序
• 在内存初始化器中(12.6.2)
• 在作业的右侧(5.17)

它们都不是三元运算符.简约的 return 1?{}:{}; 也是无效的，您想要的是不可能的.

当然，您可以显式调用构造函数 std :: list< std :: string> {} ，但是我建议写出 if - else -就像您已经做过的一样.

While developing an application, I had the following problem. I wanted to return an empty std::list<string> when a given function pointer was null, or the result of that function otherwise. This is a simplified version of my code:

typedef std::list<std::string> (*ParamGenerator)();

std::list<std::string> foo() {
    /* ... */
    ParamGenerator generator = ...;
    if(generator)
        return generator();
    else
        return {};
}

However, I usually like to use the ternary (?:) operator in these cases, so I tried using it this way (as usual):

return generator ? generator() : {};

But got this error:

somefile.cpp:143:46: error: expected primary-expression before ‘{’ token
somefile.cpp:143:46: error: expected ‘;’ before ‘{’ token

Does this mean I can't use the ternary operator to return objects created using their constructor from an initializer_list? Is there any particular reason for that?

解决方案

Standard writes in 8.5.4.1: List-initialization

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 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)

Nothing of them is a ternary operator. The more minimalistic return 1?{}:{}; is invalid too, what you want is impossible.

Of course you can explicitly call the constructor std::list<std::string>{}, but I would recommend to write out the if-else-block as you already did.

这篇关于来自initializer_list的三元运算符+ C ++ 11构造函数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！