【本文连接】
http://www.cnblogs.com/hellogiser/p/user_initialization_list.html
【分析】
初始化列表和构造函数内的赋值语句有何区别？
(1)对于built-in数据类型，如int long，指针等，初始化列表和构造函数内的赋值语句效果和效率一样。
(2)对于user-defined数据类型，比如class对象等，初始化列表和构造函数内的赋值语句效果是一样的，但是效率却有很大的差异。
如果一个Class1内部有Class2的一个对象Class2 obj，并且通过Class2 &robj来初始化，那么
(2.1)对于执行初始化列表的obj(robj)部分，只会调用Class2的copy constructor
(2.2)对于构造函数内的赋值obj = robj语句，在执行构造函数体之前，会调用Class2的default constructor来初始化obj，然后执行obj = robj语句，调用Class2的copy assignment。
也就是说，初始化列表比构造函数内赋值要更高效。
【const数据成员】
对于Class的const或者ref数据成员，只能通过初始化列表初始化，不能够赋值。
【代码1】
C++ Code
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#include <stdio.h>
#include <stdlib.h> #include <string.h> #include <iostream> using namespace std; class Food { public: Food(int v = 10): m_v(v) { puts("default consturctor"); } Food(const Food &other) { puts("copy consturctor"); this->m_v = other.m_v; } Food &operator =(const Food &other) { puts("copy assignment"); if(this == &other) return *this; this->m_v = other.m_v; return *this; } private: int m_v; }; class Dog { public: /* for User Intialization List only copy constructor was called * */ Dog(int w, Food &f): weight(w), food(f), LEGS(4) { cout << weight << endl; // 100 } private: int weight; const int LEGS; Food food; }; void test_case1() { Food f; Dog d(111, f); puts("-----------------"); } int main() { test_case1(); return 0; } /* * default consturcor * copy constructor * 111 * */ |
【代码2】
C++ Code
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#include <stdio.h>
#include <stdlib.h> #include <string.h> #include <iostream> using namespace std; class Food { public: Food(int v = 10): m_v(v) { puts("default consturctor"); } Food(const Food &other) { puts("copy consturctor"); this->m_v = other.m_v; } Food &operator =(const Food &other) { puts("copy assignment"); if(this == &other) return *this; this->m_v = other.m_v; return *this; } private: int m_v; }; class Dog2 { public: /* for assignment inside {} of constructor default constructor + copy assignment were called * */ Dog2(int w, Food &f): LEGS(4) { cout << weight << endl; // -3174682 (has not initialized so a random number) weight = w; food = f; } private: int weight; const int LEGS; Food food; }; void test_case2() { Food f; Dog2 d(99, f); puts("-----------------"); } int main() { test_case2(); return 0; } /* * default constructor * default construcotr * -876545120 * copy assignment * */ |