C++构造函数的初始化列表详解
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2022-03-04 13:59:45
目录1.问题2.解决方法(初始化列表)3.顺序问题总结1.问题class a {private:int m_a;public:a(int a) {cout << "a(int a).......
1.问题
class a { private: int m_a; public: a(int a) { cout << "a(int a)......." << endl; m_a = a; } void print() { cout <<"m_a=" << m_a << endl; } }; class b { private: int m_b; a m_a1; a m_a2; public: b(a& a1,a& a2, int b) { m_b = b; m_a1(a1);//此处调用a的拷贝函数会报错 m_a2(a2);//此处调用a的拷贝函数会报错 } };
错误:
2.解决方法(初始化列表)
将class b构造函数改写为:
public: b(a& a1,a& a2, int b) :m_a1(a1),m_a2(a2)//构造函数的初始化列表 { m_b = b; } };
完整代码如下:
#include <iostream> using namespace std; class a { private: int m_a; public: a(int a) { cout << "a(int a)......." << endl; m_a = a; } void print() { cout <<"m_a=" << m_a << endl; } a(const a& another) { m_a = another.m_a; } ~a() { cout << "~a()......" << endl; } }; class b { private: int m_b; a m_a1; a m_a2; public: b(a& a1,a& a2, int b) :m_a1(a1),m_a2(a2)//构造函数的初始化列表,调用拷贝构造 { cout << "b(a& a1,a& a2, int b)......." << endl; m_b = b; } ~b() { cout << "~b()......." << endl; cout << "m_b=" << m_b << endl; cout << "a m_a1" << endl; m_a1.print(); cout << "a m_a2" << endl; m_a2.print(); } }; int main(int argc, char** argv) { a a1(1), a2(2); b b1(a1, a1, 3); }
运行结果:
3.顺序问题
构造对象成员的顺序跟初始化列表的顺序无关,而是跟成员对象定义的顺序有关。(面试会问)
例子:
class a { private: int m_a; public: a(int a) { cout << "a(int a)......." <<a<< endl; m_a = a; } void print() { cout <<"m_a=" << m_a << endl; } a(const a& another) { m_a = another.m_a; } ~a() { cout << "~a()......"<< endl; } }; class b { private: int m_b; a m_a2; a m_a1; public: b(int a1, int a2, int b) :m_a1(a1), m_a2(a2)//调用有参构造函数 { cout << "b(int a1, int a2, int b)......." << endl; m_b = b; } ~b() { cout << "~b()......." << endl; } }; int main(int argc, char** argv) { b b2(1, 2, 3); }
结果:
跟下面顺序有关:
private: a m_a2; a m_a1;
跟下面顺序无关:
b(int a1, int a2, int b) :m_a1(a1), m_a2(a2)//调用有参构造函数
总结
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