使用C ++ 11的成员初始值设定项列表中的初始值设定项列表语法 [英] Initializer list syntax in member initializer list using C++11

问题描述

我一直在浏览' A C ++浏览'，而Bjarne使用c构造函数中的成员初始化中的++ 11初始化程序列表功能，如下所示(使用大括号):

I've been going through 'A Tour of C++' and Bjarne uses the the c++11 initializer list feature in member initialization in a constructor, like so (using curly brackets):

A a;
B b;
Foo(Bar bar):
  a{bar.a}, b{bar.b}
{}

但是，此文件在c ++ 11之前无法编译.与旧成员初始值设定项列表有什么区别(使用圆括号):

This, however doesn't compile prior to c++11. What is the difference with the old member initializer list (using round brackets):

Foo(Bar bar):
  a(bar.a), b(bar.b)
{}

那有什么区别?什么时候应该优先选择另一个?

So what is the difference and when should one be preferred over the other?

推荐答案

那有什么区别?

So what is the difference?

圆括号仅适用于非类类型，或者具有适当构造函数的类型可用于括号中的参数.

Round brackets only work for non-class types, or types with a suitable constructor for the number of arguments in the brackets.

花括号适合这些对象，也适合聚合-简单的struct或没有构造函数的数组类型.因此，以下方法将起作用:

Squiggly braces work for these, and also for aggregates - simple struct or array types with no constructor. So the following will work:

struct {
    int a,b;
} aggregate;
int array[2];

Foo() : aggregate{1,2}, array{3,4} {}

最后，花括号将匹配采用适当类型的initializer_list的构造函数，而不是带有参数的构造函数以匹配参数.例如:

Finally, braces will match a constructor taking a suitably-typed initializer_list, rather than a constructor with parameter(s) to match the arguments. For example:

std::vector<int> v1;
std::vector<int> v2;

Foo() :
    v1(10,2),   // 10 elements with value 2
    v2{10,2}    // 2 elements with value 10,2
{}

何时应该优先选择另一个?

when should one be preferred over the other?

如果希望更清楚地表明初始化是使用构造函数而不是聚合或initializer_list，则最好使用圆括号；或强制使用特定的构造函数.

Prefer round brackets if you want to make it clearer that the initialisation is using a constructor rather than aggregate or initializer_list; or to force use of a specific constructor.

在需要以其他方式不支持的初始化形式时，请使用大括号；或者当您只希望初始化做正确的事"时.

Prefer braces when you need a form of initialisation not otherwise supported; or when you just want the initialisation to "do the right thing".

在两者都做相同事情的情况下，选择主要是出于审美目的.

In the cases where both do the same thing, the choice is largely aesthetic.

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