c++ method const, return type const 7 minutes, maybe 5 minutes

const

const qualifier makes the object immutable. (Read day-10)

Method const

A const qualifier is placed after the method name and a pair of (), it will make the object immutable inside the scope of the current method.

class my_class
{
public:
	void fz_kp() const	// Method const
	{
	}
	void kp_fz()
	{
	}
};

The above const qualifer makes the current object immutable inside of the method .fz_kp(),

it can not make the object immutable outside of the method .fz_kp(), such as .kp_fz() .

To quickly understand it, the method const qualifier make the "*this" const .

Return type const

Don't be confused by return type const and method const, the return type const qualifier is paced before the method_name() .

class my_class
{
public:
	const int kf_pz()	// Return type const: The value it returned is const.
	{
		return 5;
	}
};

Method const methods calling each other

I name method that is qualified by method const as const method now.

why?

This is an unofficial understanding:

You can insert immutable things into mutable things, but you can not insert mutable things into immutable things.

class my_class
{
public:
	void f_a() const {}
	void f_b() {}
	void f_c() const
	{
		f_a();	// OK
		//f_b();	// not allowed
	}
	void f_d()
	{
		f_a();	// OK
		f_b();	// OK
	}
};

int main()
{
	const auto obj = my_class{};
	obj.f_a();	// OK
	//obj.f_b();	// ERROR, not allowed

	auto obj2 = my_class{};
	obj2.f_a();	// OK
	obj2.f_b();	// OK
}

Written on Oct 31, 2024

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