Overloaded Functions

Functions that have the same name but different parameter lists and that appear in the same scope are overloaded

• It is an error for two functions to differ only in terms of their return types

⚠️ 别用 cast 来 call func，如果真的需要这么做，你应该改进的函数。

Functions Matching

除了看最基本的参数个数，参数类型之外，当遇到 conversion时，the viable function is：other than error

• 它的 argument 契合度不比别的函数的差（大于等于）
• 它的 argument 契合度至少有一个比别的函数的强（大于）

Conversions are Ranked as Follows: Implicit Conversions

1. exact match
   • 就是完全一样
   • 数组或函数转变为对应指针
   • top-level const is added to or discarded from
2. low-level const conversion
   • 当两个都有的时候，会区分出 the best match
3. promotion
   • 注意 interal type 总是会先变成 int 当不是 exact match。所以 'a' int short 会匹配 int
   • convention 之间是没有优先级，所以 3.14 long float is ambiguous.
4. arithmetic or pointer conversion
5. class-type conversion

• A parameter that has a top-level const is indistinguishable from one without a top-level const. (But low-level const is distinguishable. More details in Funcs Matching.)
  虽然当只有const的参数表时，nonconst也能match，但是当两个都有的时候，就会区分了（不知道是不是跟那个“tmp”有关。猜对了！就是和这个有关）

const_cast and Overloading

🔗 Type Conversions

/* example */
const string &shorterString(const string &s1, const string &s2){
	return s1.size() <= s2.size() ? s1 : s2;
}
 
string &shorterString(string &s1, string &s2){
	auto &tmp = shorterString(const_cast<const string&>(s1), 
							  const_cast<const string&>(s2));
	return const_cast<string&>(r);
}

• 如果我们不用const，那我们就传不进const的object，也传不进literal
• 如果只用const，那传进去的是nonconst结果出来的是const，这多不好！
• 这个方法确保了：如果进去的是nonconst，那么cast on or away the const is legal. 如果进去的是const，那么不会运行第二个函数。所以是安全的

Overloading and Scope

其实跟变量的情况非常类似

string read();    // note that the name of a func is also a name
void print(const string &);
void print(double);  // overload
 
void fooBar(int ival){
	bool read = false;  // new scope: hide the outer read
	string s = read();  // error: bool is not a func
 
	void print(int);    // new scope: hide the outer two func
}

• Names do not overload across scopes. (串域不会overload)
• In C++, name lookup happens before type checking.

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term	mean
candidate functions	same name and is visible at the point of the call
viable functions