【深入理解C++】隐式构造与explicit

1.隐式构造

C++ 中存在隐式构造的现象，即在某些情况下，会隐式调用单参数的构造函数。

1.1 情况一

#include 

using namespace std;

class Student
{
private:
	int age;

public:
	Student()
	{
		age = 0;
		cout << "Student() - " << this << endl;
	}
	Student(int v)
	{
		age = v;
		cout << "Student(int v) - " << this << endl;
	}
	Student(const Student& stu) // 拷贝构造函数
	{
		age = stu.age;
		cout << "Student(const Student& stu) - " << this << endl;
	}
	Student& operator=(const Student& stu) // 拷贝赋值运算符
	{
		age = stu.age;
		cout << "Student& operator=(const Student& stu) - " << this << endl;
		return *this;
	}
	~Student()
	{
		cout << "~Student() - " << this << endl;
	}
	void display()
	{
		cout << "display() - " << this << endl;
	}
};

int main()
{
	Student stu1 = 20;

	return 0;
}

1.2 情况二

#include 

using namespace std;

class Student
{
private:
	int age;

public:
	Student()
	{
		age = 0;
		cout << "Student() - " << this << endl;
	}
	Student(int v)
	{
		age = v;
		cout << "Student(int v) - " << this << endl;
	}
	Student(const Student& stu) // 拷贝构造函数
	{
		age = stu.age;
		cout << "Student(const Student& stu) - " << this << endl;
	}
	Student& operator=(const Student& stu) // 拷贝赋值运算符
	{
		age = stu.age;
		cout << "Student& operator=(const Student& stu) - " << this << endl;
		return *this;
	}
	~Student()
	{
		cout << "~Student() - " << this << endl;
	}
	void display()
	{
		cout << "display() - " << this << endl;
	}
};

void test1(Student stu)
{

}

int main()
{
	test1(30);

	return 0;
}

1.3 情况三

#include 

using namespace std;

class Student
{
private:
	int age;

public:
	Student()
	{
		age = 0;
		cout << "Student() - " << this << endl;
	}
	Student(int v)
	{
		age = v;
		cout << "Student(int v) - " << this << endl;
	}
	Student(const Student& stu) // 拷贝构造函数
	{
		age = stu.age;
		cout << "Student(const Student& stu) - " << this << endl;
	}
	Student& operator=(const Student& stu) // 拷贝赋值运算符
	{
		age = stu.age;
		cout << "Student& operator=(const Student& stu) - " << this << endl;
		return *this;
	}
	~Student()
	{
		cout << "~Student() - " << this << endl;
	}
	void display()
	{
		cout << "display() - " << this << endl;
	}
};

Student test2()
{
	return 40;
}

int main()
{
	test2();

	return 0;
}

2.explicit

上面的三个代码都隐式调用了单参数的构造函数，将一个整数类型 int 转换为类类型 Student。但是，这种隐式类型转换有些不伦不类，容易让人产生疑惑，有什么办法可以阻止这种用法呢？

答案：使用 explicit 关键字。explicit 指定构造函数或转换函数为显式，即不能用于隐式转换和复制初始化。

#include 

using namespace std;

class Student
{
private:
	int age;

public:
	Student()
	{
		age = 0;
		cout << "Student() - " << this << endl;
	}
	explicit Student(int a)
	{
		age = a;
		cout << "Student(int v) - " << this << endl;
	}
	Student(const Student& stu) // 拷贝构造函数
	{
		age = stu.age;
		cout << "Student(const Student& stu) - " << this << endl;
	}
	Student& operator=(const Student& stu) // 拷贝赋值运算符
	{
		age = stu.age;
		cout << "Student& operator=(const Student& stu) - " << this << endl;
		return *this;
	}
	~Student()
	{
		cout << "~Student() - " << this << endl;
	}
	void display()
	{
		cout << "display() - " << this << endl;
	}
};

int main()
{
	Student stu1 = 18; // 报错
	Student stu2 = { 20 }; // 报错

	return 0;
}

explicit 关键字只对带一个参数的类构造函数有效，如果类构造函数参数大于或等于两个，是不会产生隐式转换的，所以 explicit 关键字也就无效了。

但是，如果类构造函数除了第一个参数以外的其他参数都有默认值，此时等效于只有一个参数的类构造函数，那么 explicit 关键字依然有效。

#include 

using namespace std;

class Student
{
private:
	int age;
	int height;

public:
	Student()
	{
		age = 0;
		height = 0;
		cout << "Student() - " << this << endl;
	}
	explicit Student(int a, int b = 180)
	{
		age = a;
		height = b;
		cout << "Student(int v) - " << this << endl;
	}
	Student(const Student& stu) // 拷贝构造函数
	{
		age = stu.age;
		height = stu.height;
		cout << "Student(const Student& stu) - " << this << endl;
	}
	Student& operator=(const Student& stu) // 拷贝赋值运算符
	{
		age = stu.age;
		height = stu.height;
		cout << "Student& operator=(const Student& stu) - " << this << endl;
		return *this;
	}
	~Student()
	{
		cout << "~Student() - " << this << endl;
	}
	void display()
	{
		cout << "display() - " << this << endl;
	}
};

int main()
{
	Student stu1 = 18; // 报错
	Student stu2 = { 20 }; // 报错

	return 0;
}