Question

使用新的{}语法初始化对象：

int a { 123 };

有好处 - 您不会声明函数而不是错误地创建变量。我甚至听说应该习惯这样做。但看看会发生什么：

// I want to create vector with 5 ones in it:
std::vector<int> vi{ 5, 1 }; // ups we have vector with 5 and 1.

那是这个好习惯吗？有没有办法避免这样的问题？

Answer 1

使用list initialization初始化对象在适用的地方应该是首选，因为：

其他好处 list-initialization限制允许的隐式缩小转换。

特别禁止：
- 从浮点类型转换为整数类型
- 从long double转换为double或转换为float并从double转换为float，除非来源是不会发生常量表达和溢出。
- 从整数类型转换为浮点类型，除非源是可以存储其值的常量表达式完全在目标类型中。
- 从整数或无范围枚举类型转换为不能表示原始值的整数类型，除外其中source是一个常量表达式，其值可以存储完全在目标类型中。
另一个好处是免疫most vexing parse。
此外，初始化列表构造函数优先于其他构造函数可用的构造函数，默认值除外。
此外，它们可以广泛使用，所有STL容器都有初始化列表构造函数。

现在关于你的例子，我会说知识带来力量。有一个特定的构造函数用于生成5个向量（即std::vector<int> vi( 5, 1);）。

Answer 2

Frankly, the subtleties of the various initialization techniques make it difficult to say that any one practice is a "good habit."

As mentioned in a comment, Scott Meyers discusses brace-initialization at length in Modern Effective C++. He has made further comments on the matter on his blog, for instance here and here. In that second post, he finally says explicitly that he thinks the morass of C++ initialization vagaries is simply bad language design.

As mentioned in 101010's answer, there are benefits to brace-initialization. The prevention of implicit narrowing is the main benefit, in my opinion. The "most vexing parse" issue is of course a genuine benefit, but it's paltry--it seems to me that in most cases an incorrect int a(); instead of int a; would probably be caught at compile time.

But there are at least two major drawbacks:

In C++11 and C++14, auto always deduces std::initializer_list from a brace-initializer. In C++17, if there's only one element in the initialization list, and = is not used, auto deduces the type of that element; ~~the behavior for multiple elements is unchanged~~ (See the second blog post linked above for a clearer explanation, with examples.) (Edit: as pointed out by T.C. in a comment below, my understanding of the C++17 rules for auto with brace initialization is still not quite right.) All of these behaviors are somewhat surprising and (in mine and Scott Meyers' opinions) annoying and perplexing.
101010's third listed benefit, which is that initialization list constructors are preferred to all other constructors, is actually a drawback as well as a benefit. You've already mentioned that the behavior of std::vector<int> vi{ 5, 1 }; is surprising to people familiar with vector's old two-element constructor. Scott Meyers lists some other examples in Effective Modern C++. Personally, I find this even worse than the auto deduction behavior (I generally only use auto with copy initialization, which makes the first issue fairly easy to avoid).

EDIT: It turns out that stupid compiler-implementation decisions can sometimes be another reason to use brace-initialization (though really the #undef approach is probably more correct).

总是用{}初始化对象是好习惯吗？

2 个答案: