关于c ++中const_cast的问题 [英] Question about const_cast in c++

问题描述

all：
这是从Effective C ++第三版编辑中引用的

all: this is quoted from Effective C++ 3rd editiion

const_cast通常用于丢弃对象的constness 。这是唯一的C ++风格的转换可以做到这一点。

const_cast is typically used to cast away the constness of objects. It is the only C++-style cast that can do this.

我的问题是可以const_cast添加constness到一个非const对象？
实际上我写了一个小程序试图批准我的想法。

My question is can const_cast add constness to a non-const object? Actually i wrote a small programme trying to approve my thought.

class ConstTest
{
 public:

 void test() {
    printf("calling non-const version test const function \n");
}

 void test() const{
    printf("calling const version test const function \n");

} 

};
 int main(int argc,char* argv){
 ConstTest ct;
 const ConstTest cct;
 cct.test();
 const_cast<const ConstTest&>(ct).test();//it is wrong to write this statement without the '&',why

}

省略以下&'引起的错误：

Omitting the '&' incurs error below:

错误C2440：'const_cast'：无法从'ConstTest'转换为'const ConstTest'

error C2440: 'const_cast' : cannot convert from 'ConstTest' to 'const ConstTest'

const_cast可以添加constness，但似乎你必须转换为对象引用，这个引用的神奇之处是什么？

It shows that const_cast can add constness,but seems like you have to cast to a object reference, what is the magic of this reference ?

推荐答案

您不需要 const_cast 添加constness：

You don't need const_cast to add constness:

class C;
C c;
C const& const_c = c;

另一方面需要一个 const_cast

const C const_c;
C& c = const_cast<C&>(const_c);

但是如果您尝试对 c使用非const操作，行为是未定义的

but behavior is undefined if you try to use non-const operations on c.

顺便说一句，如果你不使用引用，则对象的副本是：

By the way, if you don't use a reference, a copy of the object is to be made:

C d = const_c; // Copies const_c

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