是安全bool成语在C ++ 11过时吗？ [英] Is the safe-bool idiom obsolete in C++11?

问题描述

@R的此回答。 Martinho Fernandes 表明，安全bool成语在C ++ 11中已被弃用，因为它可以被一个简单的

替换。

  explicit operator bool（）const; 
   
 
 
根据标准报价在回答§4[conv] / code>：
 
 
表达式e可以隐式转换为 T 如果且仅当声明 T t = e; 形式良好时，对于一些发明的临时变量 t （§8.5）。某些语言结构要求将表达式转换为布尔值。在这种上下文中出现的表达式 e 被称为上下文转换为 bool 如果且仅当声明 bool t（e）; 形式良好，对于一些发明的临时变量t ）。
 
 
突出显示的部分清楚地显示了隐式显式转换（在标准中称为上下文转换）为@R。 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 $ b b 
 
 
  if ， while ， code>（§6.4[stmt.select] p4 ）
 
 
二进制逻辑运算符& & 和 || （§5.14[expr.log.and / or] p1  for each）
 
 
逻辑否定运算符！（§5.3.1[expr.unary.op ] p9 ）
 
 
条件运算符？：（§5.14[expr。 cond] p1 ）
 
 
  static_assert （§7[dcl.dcl] p4 ）
 
 
  noexcept （§15.4[except.spec] / code>）
 
 
 
 
 
标题中的假设是否正确？我希望我们没有忽略任何潜在的缺点。
解决方案
是的。这是 示例，仅限于具有隐式用户定义转换的问题，而显式用户定义的转换运算符实际上因为这个问题而发生的事件，并用更清晰，更合乎逻辑的东西替换所有的safe-bool东西。
 
This answer of @R. Martinho Fernandes shows, that the safe-bool idiom is apperently deprecated in C++11, as it can be replaced by a simple
explicit operator bool() const;
according to the standard quote in the answer §4 [conv] p3:

  
An expression e can be implicitly converted to a type T if and only if the declaration T t=e; is well-formed, for some invented temporary variable t (§8.5). Certain language constructs require that an expression be converted to a Boolean value. An expression e appearing in such a context is said to be contextually converted to bool and is well-formed if and only if the declaration bool t(e); is well-formed, for some invented temporary variable t (§8.5).
The highlighted part clearly shows the "implicit explicit cast" (called "contextual conversion" in the standard) as @R. Martinho put it.

The "certain language constructs" that require that "implicit explicit cast" seem to be the following:


if, while, for (§6.4 [stmt.select] p4)
binary logical operators && and || (§5.14 [expr.log.and/or] p1 for both)
the logical negation operator ! (§5.3.1 [expr.unary.op] p9)
conditional operator ?: (§5.14 [expr.cond] p1)
static_assert (§7 [dcl.dcl] p4)
noexcept (§15.4 [except.spec] p2)


Is our assumption in the title correct? I hope we didn't overlook any potential drawbacks.
解决方案
Yes. This is the example for problems with only having implicit user-defined conversions and explicit user-defined conversion operators were practically invented because of this problem and to replace all the safe-bool stuff with something a lot cleaner and more logical.

                        
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