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請(qǐng)問(wèn)這里的const_cast是什么用法？

C++ C

慕斯709654 2022-04-22 15:15:37

const_cast不是去除底層const的作用嗎，這里怎么反倒顯示轉(zhuǎn)化為const 引用了

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Helenr

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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

1 #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 }