为什么列表初始化（使用花括号）比替代选择更好？ [英] Why is list initialization (using curly braces) better than the alternatives?

问题描述

  MyClass a1 {a}; //比其他三个更清晰，更不容易出错
 MyClass a2 = {a}; 
 MyClass a3 = a; 
 MyClass a4（a）; 
   
 
 
 
 为什么？ 
 
 
 
我找不到答案，所以让我回答自己的问题。
解决方案
基本上复制和粘贴来自Bjarne Stroustrup的C ++程序设计语言第4版：
 
 
 
 列表初始化不允许缩小.8.5.4）。也就是：
 
 
 
 
 
整数不能转换为不能容纳其值的另一个整数。例如，允许char 
到int，但不允许int到char。
 
 
浮点值不能转换为不能容纳其$ b的浮点类型$ b值。例如，允许float为double，但不能为double。
 
 
浮点值无法转换为整数类型。
 
 
整数值不能转换为浮点类型。
 
 
 
 
 
示例：
  void fun（double val，int val2）{
 
 int x2 = val; // if val == 7.9，x2 become 7（bad）
 
 char c2 = val2; // if val2 == 1025，c2 become 1（bad）
 
 int x3 {val}; //错误：可能截断（好）
 
 char c3 {val2}; //错误：可能变窄（好）
 
 char c4 {24}; // OK：24可以表示为一个char（好）
 
 char c5 {264}; //错误（假设8位字符）：264不能为
 //表示为char（好）
 
 int x4 {2.0}; // error：no double to int value conversion（good）
 
} 
  
 
 
 
 
 
 
只有 情况，其中=是首选的{}是使用 auto  
 
 
 
例如：
  auto z1 { 99}; // z1是initializer_list< int> 
 auto z2 = 99; // z2 is an int 
  
 
 
 
 
 
 
结论
 
 
 
 优先{}初始化替代品，除非你有强烈的理由不要。 
 
MyClass a1 {a};     // clearer and less error-prone than the other three
MyClass a2 = {a};
MyClass a3 = a;
MyClass a4(a);
Why? 

I couldn't find an answer on SO, so let me answer my own question.
解决方案
Basically copying and pasting from Bjarne Stroustrup's "The C++ Programming Language 4th Edition":

List initialization does not allow narrowing (§iso.8.5.4). That is:


An integer cannot be converted to another integer that cannot hold its value. For example, char
to int is allowed, but not int to char.
A floating-point value cannot be converted to another floating-point type that cannot hold its
value. For example, float to double is allowed, but not double to float.
A floating-point value cannot be converted to an integer type.
An integer value cannot be converted to a floating-point type.


Example:
void fun(double val, int val2) {

    int x2 = val; // if val==7.9, x2 becomes 7 (bad)

    char c2 = val2; // if val2==1025, c2 becomes 1 (bad)

    int x3 {val}; // error: possible truncation (good)

    char c3 {val2}; // error: possible narrowing (good)

    char c4 {24}; // OK: 24 can be represented exactly as a char (good)

    char c5 {264}; // error (assuming 8-bit chars): 264 cannot be 
                   // represented as a char (good)

    int x4 {2.0}; // error: no double to int value conversion (good)

}




The only situation where = is preferred over {} is when using auto keyword to get the type determined by the initializer.

Example:
auto z1 {99}; // z1 is an initializer_list<int>
auto z2 = 99; // z2 is an int




Conclusion

Prefer {} initialization over alternatives unless you have a strong reason not to.

                        
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