April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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176
｜
第
5
章
explicit Person(int idx)
//
同前
: name(nameFromIdx(idx)) {}
…
private:
std::string name;
};
由于
std::string
型别并没有只接受单个整型形参的构造函数，所有
int
或者类
int
型别（例如，
std::size_t
，
short
，
long
）的实参都会汇集到接受
int
型别的那个
构造函数重载版本的调用。类似地，所有
std::string
型别的实参（包括可以构造出
std::string
型别对象之物，例如字面量“
Ruth
”）都会被传递给接受
std::string
型别的那个构造函数。综上，不会有让调用者出乎意料的情况。不过也许你可能会说
一些人会传递
NULL
和
0
来表示空指针，从而意外调用接受
int
型别的那个重载版本。
但是，这些人应该参阅条款
8
，而且要一遍遍地读，直到他们一想到要使用
NULL
和
0
表示空指针，就会立刻退缩为止。
标签分派
无论是传递左值常量还是传值，都不支持完美转发。如果使用万能引用的动机就是为
了实施完美转发，那就只能采用万能引用，别无他途。然而，我们也不想舍弃重载。
那么，如果一不想放弃重载，二不想放弃万能引用，那又该如何避免依万能引用型别
进行重载呢？
其实解决之道并非那么难。重载函数在调用时的决议，会考察所有重载版本的形参，
以及调用端传入的实参，然后选择全局最佳匹配的函数，这需将所有的形参／实参组
合都考虑在内。一个万能引用形参通常会导致的后果是无论传入了什么都给出一个精
确匹配结果，不过，如果万能引用仅是形参列表 ...
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