以下示例摘自cppreference:
#include <iostream>
#include <memory>
#include <experimental/propagate_const>
struct X
{
void g() const { std::cout << "g (const)\n"; }
void g() { std::cout << "g (non-const)\n"; }
};
struct Y
{
Y() : m_ptrX(std::make_unique<X>()) { }
void f() const
{
std::cout << "f (const)\n";
m_ptrX->g();
}
void f()
{
std::cout << "f (non-const)\n";
m_ptrX->g();
}
std::experimental::propagate_const<std::unique_ptr<X>> m_ptrX;
};
int main()
{
Y y;
y.f();
const Y cy;
cy.f();
}
我想进一步确保指针(m_ptrX
)地址不可修改,所以我将声明更改为
std::experimental::propagate_const<const std::unique_ptr<X>> m_ptrX;
但是它不起作用,gcc 9报告以下错误(有关详细信息,请参见here)
g++ -std=c++2a -pthread -O2 -Wall -Wextra -pedantic -pthread -pedantic-errors main.cpp -lm -latomic -lstdc++fs && ./a.out
In file included from main.cpp:3:
/usr/local/include/c++/9.2.0/experimental/propagate_const: In instantiation of 'constexpr std::experimental::fundamentals_v2::propagate_const<_Tp>::element_type* std::experimental::fundamentals_v2::propagate_const<_Tp>::get() [with _Tp = const std::unique_ptr<X>; std::experimental::fundamentals_v2::propagate_const<_Tp>::element_type = X]':
/usr/local/include/c++/9.2.0/experimental/propagate_const:205:13: required from 'constexpr std::experimental::fundamentals_v2::propagate_const<_Tp>::element_type* std::experimental::fundamentals_v2::propagate_const<_Tp>::operator->() [with _Tp = const std::unique_ptr<X>; std::experimental::fundamentals_v2::propagate_const<_Tp>::element_type = X]'
main.cpp:24:15: required from here
/usr/local/include/c++/9.2.0/experimental/propagate_const:225:25: error: invalid conversion from 'const element_type*' {aka 'const X*'} to 'std::experimental::fundamentals_v2::propagate_const<const std::unique_ptr<X> >::element_type*' {aka 'X*'} [-fpermissive]
225 | return __to_raw_pointer(_M_t);
| ~~~~~~~~~~~~~~~~^~~~~~
| |
| const element_type* {aka const X*}
那么,如果我不想实现像immutable_unique_ptr
这样的模板,那么达到此效果的正确方法是什么?
[通过在std::unique_ptr
中将其变为const
似乎无法保护std::experimental::propagate_const
免受修改,但是要了解为什么,我们必须遍历std::experimental::propagate_const
的源代码,但可以在propagate_const处找到它。] >
[propagate_const
有一个私有变量,