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。

 
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,所以他們對於需要更改操作的arguments,可以使用pass-by-point的方式。
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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);
In fact it is a very strong convention in Google code that input arguments are values or 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.
Remember that most of the time input parameters are going to be specified as 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。

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