关于c++的构造函数的几个说明

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1月 172017

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　　本文为了说明基类和派生类的构造函数的声明和使用，主要是以下几点：

派生类所有构造函数都会默认调用基本的默认构造函数（没有参数的构造函数，如Base())
派生类如果想显示的调用基类的带参数的构造函数，要在派生类的初始化器处调用，而不能在构造函数的实现里调用（即不能在花括号里调用{}）

　　示例代码如下：

#include <iostream>

class Shape{
public:
	int length = 0;

public:
	Shape(){
		length = 3; //子类的所有构造函数都会调用基类的默认构造函数
	}

	Shape(int len){
		length = len;
	}
};

class Circle: public Shape{
public:
	Circle() {};
};

class Square: public Shape{
public:
	Square(int len) : Shape(len){ //在初始化器这里调用基类的相关构造函数，而不是在下面的实现里
		// Shape(len); //在这里会报错，放在, redefinition of 'len' with a different type: 'Shape' vs 'int'
	}

	Square(std::string name){}; //这个构造函数会调用Shape的默认构造函数
};

int main(int argc, char **argv) {
	Shape shape ;
	std::cout << "Shape length: " << shape.length << std::endl;

	Circle circle;
	std::cout << "Circle length: " << circle.length << std::endl;

	Square square(10);
	std::cout << "Square length: " << square.length << std::endl;

	Square mysquare("mysquare");
	std::cout << "mysquare length: " << mysquare.length << std::endl;
}

#include <iostream>

class Shape{

public:

int length = 0;

public:

Shape(){

length = 3; //子类的所有构造函数都会调用基类的默认构造函数

}

Shape(int len){

length = len;

}

};

class Circle: public Shape{

public:

Circle() {};

};

class Square: public Shape{

public:

Square(int len) : Shape(len){ //在初始化器这里调用基类的相关构造函数，而不是在下面的实现里

// Shape(len); //在这里会报错，放在, redefinition of 'len' with a different type: 'Shape' vs 'int'

}

Square(std::string name){}; //这个构造函数会调用Shape的默认构造函数

};

int main(int argc, char **argv) {

Shape shape ;

std::cout << "Shape length: " << shape.length << std::endl;

Circle circle;

std::cout << "Circle length: " << circle.length << std::endl;

Square square(10);

std::cout << "Square length: " << square.length << std::endl;

Square mysquare("mysquare");

std::cout << "mysquare length: " << mysquare.length << std::endl;

}

　　输出结果如下：

Shape length: 3
Circle length: 3
Square length: 10
mysquare length: 3

Shape length: 3

Circle length: 3

Square length: 10

mysquare length: 3

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