C++: const function arguments & return values

整理自 Thinking in C++

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Overview

• If you are passing objects by value, specifying const has no meaning to the client (it means that the passed argument cannot be modified inside the function, which is guaranteed by pass-by-value even without const).
• If you are returning an object of a user-defined type by value as a const, it means the returned value cannot be modified.
• If you are passing and returning addresses, const is a promise that the destination of the address will not be changed.

Passing const value

void f1(const int i) {
	i++; // Illegal -- compile-time error
}

因为是 pass-by-value，所以 copy 也是 const，自然不能修改。但是如果你的初衷只是 “保证原参数不变” 的话，用 const 只能说是 paranoid；如果函数编写者的期望是 “把参数 copy 设置成 const 避免错误的赋值”，那其实可以这么写：

void f2(int i) {
	const int& cri = i; // cri: reference to a const int. 你理解为 "cri 本身就是个 const int" 似乎更简单一些
	cri++; 				// Illegal -- compile-time error
}

Returning const value

For built-in types, it doesn’t matter whether you return a const value, so you should avoid confusing the client programmer and leave off the const when returning a built-in type by value.

//: C08:Constval.cpp
// Returning consts by value has no meaning for built-in types
int f3() { return 1; }
const int f4() { return 1; }

int main() {
	const int j = f3(); // Works fine
	int k = f4(); 		// But this works fine too!
} 

我觉得你理解成常数就好了，比如 3，你 const int j = 3; 和 int k = 3; 看上去一点问题都没有。

Returning a const value becomes important when you’re dealing with user-defined types. If a function returns a const class object, the return value of that function cannot be an $\texttt{lvalue}$ (that is, it cannot be assigned to or otherwise modified). For example:

//: C08:ConstReturnValues.cpp
// Constant return by value
// Result cannot be used as an lvalue
class X {
	int i;
public:
	X(int ii = 0);
	void modify();
};

X::X(int ii) { i = ii; }

void X::modify() { i++; }

const X f6() {
	return X();
}

int main() {
	//! f6() = X(1);	// CANNOT be assigned to
	//! f6().modify();	// CANNOT be modified
}