arm64 debian 上的 gcc-8 会忽略朋友运算符<吗？

Question

我想比较两组对象。对于相同的代码，我在不同的机器上得到2个不同的输出。代码用两个不同的编译器编译。x86-64gcc-11，在aarch64(raspberry pi4)我使用的机器gcc-8。我必须使用gcc-8在树莓上，因为它在官方存储库中。

有人知道为什么会这样吗?我错过什么了吗?

#include <iostream>
#include <set>
#include <memory>
class ClassA {
private:
::std::string id;
public:
explicit ClassA(::std::string id);
[[nodiscard]] ::std::string getId() const;
bool friend operator<(const ::std::shared_ptr<ClassA> &lhs, const ::std::shared_ptr<ClassA> &rhs);
};
ClassA::ClassA(::std::string id) : id{::std::move(id)} {}
::std::string ClassA::getId() const { return id; }
bool operator<(const ::std::shared_ptr<ClassA> &lhs, const ::std::shared_ptr<ClassA> &rhs) {
auto r = (lhs->id < rhs->id);
::std::cout << ::std::boolalpha << "Comparing lhs->id " << lhs->id << ", with rhs->id " << rhs->id
<< ". The result is " << r << ::std::endl;
return (lhs->id < rhs->id);
}
class ClassB {
public:
::std::set<::std::shared_ptr<ClassA>> members;
void add(::std::shared_ptr<ClassA> a);
};
void ClassB::add(::std::shared_ptr<ClassA> a) {
members.emplace(::std::move(a));
}
int main() {
::std::cout << "Create first set:" << ::std::endl;
auto firstContainer = ::std::set<::std::shared_ptr<ClassA>>{::std::make_shared<ClassA>("_3"),
::std::make_shared<ClassA>("_5")};
::std::cout << "Create second set:" << ::std::endl;
auto secondContainer = ::std::set<::std::shared_ptr<ClassA>>{::std::make_shared<ClassA>("_5"),
::std::make_shared<ClassA>("_3")};
auto b1 = ::std::make_shared<ClassB>();
auto b2 = ::std::make_shared<ClassB>();
::std::cout << "Fill first ClassB instance:" << ::std::endl;
for (const auto &r: firstContainer) {
b1->add(r);
}
::std::cout << "Fill second ClassB instance:" << ::std::endl;
for (const auto &r: secondContainer) {
b2->add(r);
}
auto result = ::std::equal(b1->members.begin(), b1->members.end(), b2->members.begin(),
[](const ::std::shared_ptr<ClassA> lhs, ::std::shared_ptr<ClassA> rhs) -> bool {
return lhs->getId() == rhs->getId();
});
::std::cout << ::std::boolalpha << "The result is: " << result << ::std::endl;
::std::cout << "First ClassB members" << ::std::endl;
for (const auto &r: b1->members) {
::std::cout << "Id " << r->getId() << ::std::endl;
}
::std::cout << "Second ClassB members" << ::std::endl;
for (const auto &r: b2->members) {
::std::cout << "Id " << r->getId() << ::std::endl;
}
return 0;
}

x86 - 64输出。

Create first set:
Comparing lhs->id _3, with rhs->id _5. The result is true
Comparing lhs->id _5, with rhs->id _3. The result is false
Create second set:
Comparing lhs->id _5, with rhs->id _3. The result is false
Comparing lhs->id _3, with rhs->id _5. The result is true
Comparing lhs->id _3, with rhs->id _5. The result is true
Fill first ClassB instance:
Comparing lhs->id _5, with rhs->id _3. The result is false
Comparing lhs->id _3, with rhs->id _5. The result is true
Comparing lhs->id _5, with rhs->id _3. The result is false
Fill second ClassB instance:
Comparing lhs->id _5, with rhs->id _3. The result is false
Comparing lhs->id _3, with rhs->id _5. The result is true
Comparing lhs->id _5, with rhs->id _3. The result is false
The result is: true
First ClassB members
Id _3
Id _5
Second ClassB members
Id _3
Id _5

aarch64输出:

Create first set:
Create second set:
Fill first ClassB instance:
Fill second ClassB instance:
The result is: false
First ClassB members
Id _3
Id _5
Second ClassB members
Id _5
Id _3

Answer 1

问题就在这里:

bool operator<(const ::std::shared_ptr<ClassA> &lhs, const ::std::shared_ptr<ClassA> &rhs)

共享指针有自己的比较操作符，用于比较存储指针。还有owner_less用于比较拥有的指针(一个特殊构造的共享指针可以拥有一个对象，但指向另一个对象，例如，它可以指向拥有对象的成员)。

如果你需要比较指向对象，你应该写一个比较器来做这件事，并把它作为第二个模板参数传递给set。如:

struct my_ClassA_id_less {
bool operator() (const ::std::shared_ptr<ClassA> &lhs, const ::std::shared_ptr<ClassA> &rhs) {
// your comparison code here, as for operator<
}
};
::std::set<std::shared_ptr<ClassA>, my_ClassA_id_less> my_set;