c++ - 为什么 clang 拒绝 gcc 接受的这个 unordered_set 定义？

Question

我想用我自己的哈希函数测试 unordered_set:

#include<unordered_set>
#include<iostream>
#include<functional>
using namespace std;
struct node{
    size_t value;
    bool operator == (const node& n){return value == n.value;}
};
size_t h(const node& n){
    return n.value;
}
int main(){
    unordered_set<node, std::function<size_t(const node&)>> s2(3,h);//failed
    return 0;
}

我尝试编译它，但 clang 给出了大量错误:

clang++ m.cpp -std=c++11
In file included from m.cpp:1:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_set:324:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/functional:659:21: error: invalid operands to binary
    expression ('const node' and 'const node')
        {return __x == __y;}
                ~~~ ^  ~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:2175:32: note: in instantiation of member
    function 'std::__1::equal_to<node>::operator()' requested here
                            key_eq()(__cp->__value_, __np->__next_->__value_);
                            ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:2121:9: note: in instantiation of member function
    'std::__1::__hash_table<node, std::__1::function<unsigned long (const node &)>, std::__1::equal_to<node>, std::__1::allocator<node> >::__rehash' requested here
        __rehash(__n);
        ^

我不太明白这里的错误信息，你能帮忙告诉我如何修复我的代码吗？

Answer 1

虽然 Baum mit Augen 已经告诉您问题所在，但我认为最好也解释一下如何从错误消息中找出更多信息。

clang++ m.cpp -std=c++11
In file included from m.cpp:1:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_set:324:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/functional:659:21: error: invalid operands to binary
    expression ('const node' and 'const node')
        {return __x == __y;}
                ~~~ ^  ~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:2175:32: note: in instantiation of member
    function 'std::__1::equal_to::operator()' requested here
                            key_eq()(__cp->__value_, __np->__next_->__value_);
                            ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:2121:9: note: in instantiation of member function
    'std::__1::__hash_table, std::__1::equal_to, std::__1::allocator >::__rehash' requested here
        __rehash(__n);
        ^

This first part is telling you that there is an error comparing a const node to another const node. At this point, you will need to exercise your own judgement to determine whether you should be able to compare two const nodes.

The answer here is yes. At which point you can simplify your code to take unordered_set out of the equation, and get the compiler to give you more information about the problem:

#include<cstddef>
using namespace std;
struct node{
    size_t value;
    bool operator == (const node& n){return value == n.value;}
};
int main(){
    const node a{}, b{};
    a == b;
}

如果您尝试编译它，clang 会为您提供更多详细信息:

error: invalid operands to binary expression ('const node' and 'const node')
        a == b;
        ~ ^  ~
note: candidate function not viable: 'this' argument has type 'const node', but method is not marked const
        bool operator == (const node& n){return value == n.value;}
             ^

“method is not marked const”告诉你到底是什么问题。要修复它，就像 Baum mit Augen 的回答一样，标记方法 const。

另一方面，如果答案是“不，你不应该能够比较两个 const node 对象”，那么问题就是“为什么 unordered_set 比较两个 const node 对象以及如何停止它”。为此，初始编译器消息的其余部分将告诉您哪些部分导致了该比较。你必须从上到下，在每一步都弄清楚“这应该行得通吗？”如果是，弄清楚为什么它不起作用。如果不是，找出导致尝试的原因。