如果在一行中声明了两个对象，它们的构造顺序是什么？

Question

假设 class 被定义为

class A {
//.....
};

现在我正在创建两个对象

A a,b;

a 和 b 的创建顺序是什么？是标准定义的吗？

Answer 1

a 将首先创建，然后 b.

在这种情况下，逗号 将用作分隔符 而不是运算符。

例如来自维基百科：

    /**
      *  Commas act as separators in this line, not as an operator.
      *  Results: a=1, b=2, c=3, i=0
      */
     int a=1, b=2, c=3, i=0;

Answer 2

顺序为书面顺序，从左到右。此外，它不是逗号运算符，而只是声明符列表。当使用用户定义的逗号运算符时，顺序实际上是未指定的。

见comma operator and declarators。

Answer 3

来自 8 个声明者 [dcl.decl] 3:

Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.

接着说

A declaration with several declarators is usually equivalent to the corresponding sequence of declarations each with a single declarator. That is T D1, D2, ... Dn; is usually equivalent to T D1; T D2; ... T Dn; where T is a decl-specifier-seq and each Di is an init-declarator. An exception occurs when a name introduced by one of the declarators hides a type name used by the decl-specifiers, so that when the same decl-specifiers are used in a subsequent declaration, they do not have the same meaning.

你可以说它们是从左到右构造的。

Answer 4

C++ 规范第 8 章 [[=22=]]，说：

Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself. (100)

脚注 (100) 继续说：

(100) A declaration with several declarators is usually equivalent to the corresponding sequence of declarations each with a single declarator. That is
T D1, D2, ... Dn;
is usually equivalent to
 T D1; T D2; ... T Dn;

...然后列出一些例外情况，none 其中适用于这种简单的情况。

所以你的问题的答案是对象是按照你列出它们的顺序构造的。不，它不是逗号运算符。

Answer 5

标准：

Declarators [dcl.decl]:
Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.

示例：

class A {
public:
    A(std::string const &s): name(s) 
    { 
        std::cout << "I am " << name << '\n'; 
    }
    std::string name;
};

auto main() -> int
{
    A a("a"), b("b");
}

输出：

I am a
I am b

如果在一行中声明了两个对象，它们的构造顺序是什么？

If two objects are declared in a single line, in which order are they constructed?

c++

language-lawyer

declaration

object-construction