英文:
Default member initialization best practice
问题
在以下代码中初始化成员变量是否有性能差异？我曾认为应该使用“第二”种方法，因为它将变量初始化为2，而“第一”种方法将首先初始化为默认值（默认为零），然后分配为1。但我发现“第一”种方法随处可见，甚至在Stack Overflow上也是如此。所以我想知道它们在C++20中是否不再有差异？
class example
{
private:
int first = 1;
int second { 2 };
}
英文:
Is there a performance difference in initializing the member variables in the following code? I used to think I should use the 'second' approach, as it initializes the variable to 2 while first will be initialzed to the default value (which will be zero) and then assigned to 1. But I am seeing the first approach everywhere, also at stackoverflow. So I wonder if they don't make a difference now in c++20?
class example
{
private:
int first = 1;
int second { 2 }
};
答案1
得分: 1
你特定情况下没有性能差异。然而，如果成员变量类型只定义了explicit构造函数，你可能无法使用=语法来初始化它的值。如果有可用的移动构造函数，你仍然可以执行T m_t = T{}，但对于非平凡可移动类型，这可能会伴随一些额外的计算。此外，如果没有可用的移动构造函数，那将调用复制构造函数（如果可用），对于非平凡可复制类型，与移动构造函数相比，通常会对性能产生更大的影响。
英文:
There is no performance difference in you specific case. However, if the member variable type has only explicit constructors defined, you might not be able to use the = syntax to initialize it with a value. You might be still able to do T m_t = T{} if there is a move constructor available, but that could come with some additional computations for non-trivially-movable types. Moreover, if no move constructor is available, that would call the copy constructor (if it is available), and for non-trivially-copyable types that one has usually some bigger performance impact compared to the move one.
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