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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。代码是用两个不同的编译器编译的。在x86-64机器上我使用gcc-11，在aarch64 ( raspberry pi4 ) 机器上我使用gcc-8。我必须在树莓上使用gcc-8 ，因为它在官方存储库中。

有谁知道为什么会这样？我错过了什么吗？

#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

问题在这里：

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

共享指针有自己的比较运算符，用于比较存储的指针。还有owner_less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的对象，你应该写一个比较器来做这件事，并将它作为第二个模板参数传递给集合。像：

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;
4

1 回答 1

0

问题在这里：

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

共享指针有自己的比较运算符，用于比较存储的指针。还有owner_less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，例如它可以指向拥有的对象的成员）。

如果你需要比较指向的对象，你应该写一个比较器来做这件事，并将它作为第二个模板参数传递给集合。喜欢：

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;
于 2021-09-08T13:18:19.283 回答