什么时候应该使用直接初始化和复制初始化？ [英] When should you use direct initialization and when copy initialization?

问题描述

它只是偏好还是有特定的情况下，一个是必要的？我参考以下变量进行初始化

Is it simply preference or are there specific instances where one is necessary over another? I'm refering to the following variants for initialization

T t(e); // direct initialization
T t = e; // copy initialization

推荐答案

describe不是隐式和显式赋值，但是：

The actual names of the things you describe is not implicit and explicit assignment but :

• 复制初始化： T x = a;


• 直接初始化： T x（a）;

• Copy-initialization : T x = a;
• Direct-initialization : T x(a);

它们是不等效的，特别是在需要转换的上下文中，例如 T 类类型和 a 是一个不同的类型（参见Alf注释的上下文，甚至不涉及转换的例子）。考虑下面的代码：

They are not equivalent, most notably in contexts where a conversion is required, for example when T is of class type and a is of a different type (see Alf comment for examples of contexts which don't even involve conversion). Consider the following code :

class Test
{
public:
    explicit Test(int i) { /* ... */ }
};

int main()
{
    Test t(0);  // OK : calls Test::Test(int)
    Test u = 0; // KO : constructor is marked explicit
}

）：

• 对于直接初始化和复制初始化，其中源类型与类的相同或类的派生类目标，构造函数


• 对于其他复制初始化情况，如 main 我的示例，考虑用户定义的转换顺序。由于显式关键字不允许使用 Test 构造函数进行隐式转换，因此第二行无法编译。

• For direct-initialization and copy-initialization where the source type is the same as, or a derived class of, the class of destination, constructors are considered
• For other copy-initialization cases, like the second line of main in my example, user-defined conversion sequence are considered. As the use of the Test constructor for implicit conversion was disallowed by the explicit keyword, the second line fails to compile.

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