basic_string类是否真的具有采用多个参数的复制构造函数，或者仅仅是构造函数? [英] Does basic_string class really have copy constructor that takes more than one parameter or is it just constructor?

问题描述

我正在阅读>为什么复制构造函数会有多个参数?.

公认的答案是:

旧的std::basic_string确实也有一个:

basic_string(const basic_string& s, 
         size_type pos = 0, size_type n = npos)

但是 http://www.cplusplus.com/reference/string/basic_string/basic_string/表示:

basic_string (const basic_string& str, size_type pos, size_type len = npos,
              const allocator_type& alloc = allocator_type());

以上不是复制构造函数，而是子字符串构造函数，用于复制str从字符位置pos开始并跨越len个字符的部分.

The above isn't a copy constructor but substring constructor that copies the portion of str that begins at the character position pos and spans len characters.

C ++标准部分说:

C++ standard section says that:

如果类X的非模板构造函数是X的副本构造函数， 第一个参数的类型为X&，const X& ;、易失性X&或const volatile X& ;，或者没有其他参数或其他所有参数 参数具有默认参数

A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments

那么，该链接接受的答案是否不正确?这真的是sub-string的basic_string类构造函数吗?我已经在C ++ 98，C ++ 11&中检查了此原型.链接&上的C ++ 14规范全部显示相同.

So, Is the accepted answer from that link is incorrect? Is this really basic_string class constructor for sub-string? I've checked prototype for this in C++98, C++11 & C++14 specification on the link & all shows the same.

推荐答案

(以前)C ++ 11国际标准[basic.string] p5中basic_string类模板的规范包含以下两个构造函数(其他):

The specification of the basic_string class template in the (former) C++11 International Standard [basic.string]p5 contains the following two constructors (among others):

basic_string(const basic_string& str);
// ...
basic_string(const basic_string& str, size_type pos, size_type n = npos,
             const Allocator& a = Allocator());

第一个显然是副本构造函数，第二个显然不是副本构造函数.请注意，规范中没有没有构造函数，其中pos具有默认参数.

The first one clearly is a copy constructor, the second one is no copy constructor. Note that there is no constructor in the spec where pos has a default argument.

在C ++ 03，C ++ 14和C ++ 1z中，情况基本上相同.在C ++ 98中，这两个构造器确实是一个构造器:

The situation is essentially the same in C++03, C++14 and C++1z. In C++98, those two constructors have indeed been a single one:

 basic_string(const basic_string& str, size_type pos = 0, size_type n = npos,
 //                                                 ~~~~
              const Allocator& a = Allocator());

但是由于 <罢工> 据我所知，允许使用C ++标准库的实现将这两个构造函数合并为一个，然后成为 copy构造:

As far as I know, an implementation of the C++ Standard Library is allowed to merge those two constructors into a single one, which then becomes a copy constructor:

 basic_string(const basic_string& str, size_type pos = 0, size_type n = npos,
 //                                                 ~~~~
              const Allocator& a = Allocator());

这具有不同的行为.如LWG 42中所述(请参阅 Shafik Yaghmour的

This has different behaviour. As explained in LWG 42 (see Shafik Yaghmour's answer), the actual copy ctor of basic_string acquires a copy of str's allocator, whereas the "substring" constructor by default uses a value-initialized new object.

_感谢塞巴斯蒂安·雷德尔指出这一点.

_{Thanks to Sebastian Redl for pointing that out.}

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