const member functions and const return values

最新推荐文章于 2022-02-07 20:00:00 发布

jsc0218

最新推荐文章于 2022-02-07 20:00:00 发布

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分类专栏： C++ Programming 文章标签： fun function class object

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本文深入探讨了C++中const成员函数的使用及其与非const版本的区别，包括函数返回值的特性、const对象与非const对象调用函数时的行为差异，以及const函数如何提供更灵活的操作方式。

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Why there are two function's versions in one class(const and non-const)? Let's have a test now.

#include <iostream>
using namespace std;

class A
{
public:
	A() {}
	void fun() { cout<<"I am not const."<<endl; }
	void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
	A test1;
	test1.fun();

	const A test2;
	test2.fun();

	return 0;
}

It is clear to see that the const object will match the const version. However, what happens when we don't have the const member function?

#include <iostream>
using namespace std;

class A
{
public:
	A() {}
	void fun() { cout<<"I am not const."<<endl; }
//	void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
	A test1;
	test1.fun();

	const A test2;
	test2.fun();

	return 0;
}

An error occurs here. How about lacking of the non-const member function version?

#include <iostream>
using namespace std;

class A
{
public:
	A() {}
//	void fun() { cout<<"I am not const."<<endl; }
	void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
	A test1;
	test1.fun();

	const A test2;
	test2.fun();

	return 0;
}

It runs well. However, there is a difference--both const!

The conclusion is that const function is more flexible, while the non-const version is relatively strict.

What about the const return values? Let's see.

#include <iostream>
using namespace std;

class A
{
public:
	A() { a=0; }
	int& fun() { cout<<"I am not const."<<endl; return a; }
	const int& fun() const { cout<<"I am const."<<endl; return a; }
	void get() { cout<<"Its value is "<<a<<endl; }
private:
	int a;
};

int main()
{
	A test1;
	test1.get();
	test1.fun() = 10;
	test1.get();

	return 0;
}

The value has been changed. Notice that it is a private member value. Now try the const version.

#include <iostream>
using namespace std;

class A
{
public:
	A() { a=0; }
	int& fun() { cout<<"I am not const."<<endl; return a; }
	const int& fun() const { cout<<"I am const."<<endl; return a; }
	void get() { cout<<"Its value is "<<a<<endl; }
private:
	int a;
};

int main()
{
	A test1;
	test1.get();
	test1.fun() = 10;
	test1.get();

	const A test2;
	test2.fun() = 10;

	return 0;
}