关于C++的一个简单问题，涉及const_cast

Posted 2023-02-16

这里的const_cast是什么用法啊，const_cast不是去除底层const的作用吗，这里怎么反倒显示转化为const 引用了

参考技术A

const_cast conversion

Converts between types with different cv-qualification.

Syntax

const_cast < new_type > ( expression )    

Returns a value of type new_type.

Explanation

Only the following conversions can be done with const_cast. In particular, only const_cast may be used to cast away (remove) constness or volatility.

1) Two possibly multilevel pointers to the same type may be converted between each other, regardless of cv-qualifiers at each level.2) lvalue of any type T may be converted to a lvalue or rvalue reference to the same type T, more or less cv-qualified. Likewise, an rvalue may be converted to a more or less cv-qualified rvalue reference. The result of a reference const_cast refers to the original object if expression is a glvalue and to the materialized temporaryotherwise (since C++17).3) Same rules apply to possibly multilevel pointers to data members and possibly multilevel pointers to arrays of known and unknown bound (arrays to cv-qualified elements are considered to be cv-qualified themselves) (since C++17)4) null pointer value may be converted to the null pointer value of new_type

As with all cast expressions, the result is:

an lvalue if new_type is an lvalue reference type or an rvalue reference to function type;

an xvalue if new_type is an rvalue reference to object type;

a prvalue otherwise.

Notes

Pointers to functions and pointers to member functions are not subject to const_cast

const_cast makes it possible to form a reference or pointer to non-const type that is actually referring to a const object or a reference or pointer to non-volatile type that is actually referring to a volatile object. Modifying a const object through a non-const access path and referring to a volatile object through a non-volatile glvalue results in undefined behavior.

Keywords

const_cast

Example

Run this code

#include <iostream> struct type 
    type() :i(3) 
    void m1(int v) const 
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK as long as the type object isn't const
    
    int i;; int main() 
    int i = 3;                    // i is not declared const
    const int& cref_i = i; 
    const_cast<int&>(cref_i) = 4; // OK: modifies i
    std::cout << "i = " << i << '\\n'; 
    type t; // note, if this is const type t;, then t.m1(4); is UB
    t.m1(4);
    std::cout << "type::i = " << t.i << '\\n';     const int j = 3; // j is declared const
    int* pj = const_cast<int*>(&j);
    // *pj = 4;         // undefined behavior!     void (type::*mfp)(int) const = &type::m1; // pointer to member function//  const_cast<void(type::*)(int)>(mfp); // compiler error: const_cast does not
                                         // work on function pointers
  

Output:

i = 4 type::i = 4