使用类型转换后为何打印同一个变量会出现两个不同的结果

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代码:

const char var = 'a';
auto static_var = static_cast<char>(var);
auto const_var = const_cast<char*>(&var);

cout << "var: \t\t" << var << "\taddress:\t" << reinterpret_cast<long>(&var) << endl;
cout << "static_var: \t" << static_var << "\taddress:\t" << reinterpret_cast<long>(&static_var) << endl;

*const_var = 'b';

cout << "const_var: \t" << *const_var << "\tptr:\t\t" << reinterpret_cast<long>(const_var) << endl;
cout << "var(direct): \t" << var << "\taddress:\t" << reinterpret_cast<long>(&var) << endl;
cout << "var(ptr): \t" << *(&var) << "\taddress:\t" << reinterpret_cast<long>(&var) << endl;

输出:

var:            a       address:        6122386111
static_var:     a       address:        6122386110
const_var:      b       ptr:            6122386111
var(direct):    a       address:        6122386111
var(ptr):       b       address:        6122386111

平台:

macbook pro 13 M1
macOS 11.6
Xcode 13.0
Clang 13.0

请教大家一下, 为何最后两种输出的结果不一样, 这是什么原因导致的

8 replies • 2022-12-30 12:27:48 +08:00

lnstrument

Nov 30, 2022

1.编译器认为 var 是常量
2.const_cast 之后再写可能是 ub

[Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer
to data member resulting from a const_cast that casts away a const-qualifier may produce undefined
behavior (9.2.8.1). — end note]