問題描述

背景:我來自 Java 世界，我對 C++ 或 Qt 相當陌生.

Background: I am comming from the Java world and I am fairly new to C++ or Qt.

為了玩轉(zhuǎn)unordered_map，我編寫了以下簡單程序:

In order to play with unordered_map, I have written the following simple program:

#include <QtCore/QCoreApplication>
#include <QtCore>
#include <iostream>
#include <stdio.h>
#include <string>
#include <unordered_map>

using std::string;
using std::cout;
using std::endl;
typedef std::vector<float> floatVector;

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);
    
    floatVector c(10);
    floatVector b(10);
    
    for (int i = 0; i < 10; i++) {
        c[i] = i + 1;
        b[i] = i * 2;
    }
    
    std::unordered_map<floatVector, int> map;
    
    map[b] = 135;
    map[c] = 40;
    map[c] = 32;
  
    std::cout << "b -> " << map[b] << std::endl;
    std::cout << "c -> " << map[c] << std::endl;
    std::cout << "Contains? -> " << map.size() << std::endl;
    
    return a.exec();
}

不幸的是，我遇到了以下沒有啟發(fā)性的錯誤.甚至連行號都沒有.

Unfortunately, I am running into the folowing error which isn't inspiring. There is not even a line number.

:-1: 錯誤: collect2: ld 返回 1 個退出狀態(tài)

:-1: error: collect2: ld returned 1 exit status

知道問題的根源嗎?

推薦答案

§23.2.5，第 3 段，說:

§23.2.5, paragraph 3, says:

每個無序關(guān)聯(lián)容器由 Key 參數(shù)化，通過滿足 Hash 要求(17.6.3.4)的函數(shù)對象類型 Hash 并充當參數(shù)的哈希函數(shù)Key 類型的值，并通過一個二元謂詞 Pred 誘導 Key 類型的值的等價關(guān)系.

Each unordered associative container is parameterized by Key, by a function object type Hash that meets the Hash requirements (17.6.3.4) and acts as a hash function for argument values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key.

使用 vector 作為 Key 并且不提供明確的哈希和等價謂詞類型意味著默認的 std::hash> 和 std::equal_to> 將被使用.

Using vector<float> as Key and not providing explicit hash and equivalence predicate types means the default std::hash<vector<float>> and std::equal_to<vector<float>> will be used.

等價關(guān)系的std::equal_to很好，因為向量有一個運算符==，這就是std::equal_to 使用.

The std::equal_to for the equivalence relation is fine, because there is an operator == for vectors, and that's what std::equal_to uses.

然而，沒有 std::hash<vector<float>> 特化，這可能就是你沒有向我們展示的鏈接器錯誤所說的.您需要提供自己的哈希器才能使其正常工作.

There is however, no std::hash<vector<float>> specialization, and that's probably what the linker error you didn't show us says. You need to provide your own hasher for this to work.

編寫這種散列器的一種簡單方法是使用 boost::hash_range:

An easy way of writing such an hasher is to use boost::hash_range:

template <typename Container> // we can make this generic for any container [1]
struct container_hash {
    std::size_t operator()(Container const& c) const {
        return boost::hash_range(c.begin(), c.end());
    }
};

然后你可以使用:

std::unordered_map<floatVector, int, container_hash<floaVector>> map;

當然，如果您需要在映射中使用不同的相等語義，則需要適當?shù)囟x散列和等價關(guān)系.

Of course, if you need different equality semantics in the map you need to define the hash and equivalence relation appropriately.

_{1.但是，在對無序容器進行散列時要避免這種情況，因為不同的順序會產(chǎn)生不同的散列值，并且無法保證無序容器中的順序.}

這篇關(guān)于C++ unordered_map 在使用向量作為鍵時失敗的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！