在C++裡函式參數使用pass-by-value的話，會造成參數的傳遞成本增加，所以在C++應用程式或是component開發上比較少使用pass-by-value。
在Google C++ Style Guide裡有提到Google內部使用C++ coding的規則，從裡面也可以看到Google的rule為所有函式的參數必須是pass-by-reference，並且加上cosnt，表示傳入的參數是不可以變動的，這時你可能會有個疑問，如果函式的實作需要修改到傳入的參數怎麼辦，當然這應該是很常見的case，所以他們對於有需要更改操作的arguments，可以使用pass-by-point的方式。
All parameters passed by reference must be labeled
const.Definition:
In C, if a function needs to modify a variable, the parameter must use a pointer, eg
int foo(int *pval). In C++, the function can alternatively declare a reference parameter: int foo(int &val). Pros:Defining a parameter as reference avoids ugly code like
(*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that a null pointer is not a possible value. Cons:References can be confusing, as they have value syntax but pointer semantics.
Decision:
Within function parameter lists all references must be const:void Foo(const string &in, string *out);
const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters.However, there are some instances where using
const T* is preferable to const T& for input parameters. For example:- You want to pass in a null pointer.
- The function saves a pointer or reference to the input.
const T&. Using const T* instead communicates to the reader that the input is somehow treated differently. So if you choose const T* rather than const T&, do so for a concrete reason; otherwise it will likely confuse readers by making them look for an explanation that doesn't exist.底下是在每個使用C++為開發語言的RD應該都要讀到的Effective C++裡提到的部分，
class Person{
public:
Person();
~Person();
private:
string name;
string address;
};
class Student: public Person {
public:
Student();
~Student();
private:
string schoolName;
string schoolAddress;
};
|
若使用
bool validateStudent(Student s);
Student plato;
bool platoIsOK = validateStudent(plato);
Student裡有兩個string物件，每次建構一個Student物件，就會建構兩個string物件，而每次建購Student時也會建構一個Person物件，一個Person物件又會建構兩個string物件。
by value參數的傳遞成本: 一次Student的copy建構式 + 一次Person的copy建構式 + 四個string的建構，所以總共是「六次建構式 + 六次解構式」
|
- 儘量以pass-by-reference-to-const取代pass-by-value，前者比較高效，並可避免切割問題。
- 此規則不適用於內建型別，以及STL的迭代器和函式物件。對它們而言pass-by-value往往比較適當。
全站熱搜
留言列表
