如何在stxxl :: map中使用std :: string作为键 [英] How to use std::string as key in stxxl::map

问题描述

我正在尝试使用std :: string作为stxxl :: map中的键 对于少量的大约10-100的琴弦，插入效果很好. 但是，当尝试在其中插入大量大约100000的字符串时，我遇到了段错误.

I am trying to use std::string as a key in the stxxl::map The insertion was fine for small number of strings about 10-100. But while trying to insert large number of strings about 100000 in it, I am getting segmentation fault.

代码如下:

struct CompareGreaterString {
    bool operator () (const std::string& a, const std::string& b) const {
       return a > b;
    }
    static std::string max_value() {
       return "";
    } 
};

// template parameter <KeyType, DataType, CompareType, RawNodeSize, RawLeafSize, PDAllocStrategy (optional)>
typedef stxxl::map<std::string, unsigned int, CompareGreaterString, DATA_NODE_BLOCK_SIZE, DATA_LEAF_BLOCK_SIZE> name_map;
name_map strMap((name_map::node_block_type::raw_size)*3, (name_map::leaf_block_type::raw_size)*3);
for (unsigned int i = 0; i < 1000000; i++) { /// Inserting 1 million strings
    std::stringstream strStream;
    strStream << (i);
    Console::println("Inserting: " + strStream.str());
    strMap[strStream.str()]=i;
}

在这里我无法确定为什么我不能插入更多的字符串.插入"1377"时，我恰好遇到了分段错误.另外，我能够添加任意数量的整数作为键.我认为字符串的可变大小可能会导致此问题.

In here I am unable to identify why I am unable to insert more number of strings. I am getting segmentation fault exactly while inserting "1377". Plus I am able to add any number of integers as key. I feel that the variable size of string might be causing this trouble.

我也无法理解字符串max_value的返回值.我只是返回了一个空白字符串.

Also I am unable to understand what to return for max_value of the string. I simply returned a blank string.

推荐答案

我终于在Timo bingmann，user2079303和Martin Ba的大力帮助下找到了解决我的问题的方法.谢谢.

I have finally found the solution to my problem with great help from Timo bingmann, user2079303 and Martin Ba. Thank you.

我想和你分享.

首先，stxxl仅支持POD.这意味着它仅存储固定大小的结构.因此std :: string不能为键. stxxl :: map可用于大约100-1000个字符串，因为它们包含在物理内存本身中.当插入更多字符串时，它必须在磁盘上写，这在内部造成了一些问题.

Firstly stxxl supports POD only. That means it stores fixed sized structures only. Hence std::string cannot be a key. stxxl::map worked for about 100-1000 strings because they were contained in the physical memory itself. When more strings are inserted it has to write on disk which is internally causing some problems.

因此，我们需要使用char []使用固定字符串，如下所示:

Hence we need to use a fixed string using char[] as follows:

static const int MAX_KEY_LEN = 16;

class FixedString { 
public:
    char charStr[MAX_KEY_LEN];

    bool operator< (const FixedString& fixedString) const {
        return std::lexicographical_compare(charStr, charStr+MAX_KEY_LEN,
            fixedString.charStr, fixedString.charStr+MAX_KEY_LEN);
    }

    bool operator==(const FixedString& fixedString) const {
        return std::equal(charStr, charStr+MAX_KEY_LEN, fixedString.charStr);
    }

    bool operator!=(const FixedString& fixedString) const {
        return !std::equal(charStr, charStr+MAX_KEY_LEN, fixedString.charStr);
    } 
};

struct comp_type : public std::less<FixedString> {
    static FixedString max_value()
    {
        FixedString s;
        std::fill(s.charStr, s.charStr+MAX_KEY_LEN, 0x7f);
        return s;
    } 
};

请注意，主要所有运算符(()，==，！=)必须被覆盖，所有stxxl :: map函数才能正常工作 现在，我们可以为地图定义fixed_name_map，如下所示:

Please note that all the operators mainly((), ==, !=) need to be overriden for all the stxxl::map functions to work Now we may define fixed_name_map for map as follows:

typedef stxxl::map<FixedString, unsigned int, comp_type, DATA_NODE_BLOCK_SIZE, DATA_LEAF_BLOCK_SIZE> fixed_name_map;
fixed_name_map myFixedMap((fixed_name_map::node_block_type::raw_size)*5, (fixed_name_map::leaf_block_type::raw_size)*5);

现在程序可以正常编译并且可以接受大约10 ^ 8个字符串，而不会出现任何问题. 我们也可以像std :: map一样使用myFixedMap. {例如:myFixedMap [fixedString] = 10}

Now the program is compiling fine and is accepting about 10^8 strings without any problem. also we can use myFixedMap like std::map itself. {for ex: myFixedMap[fixedString] = 10}

这篇关于如何在stxxl :: map中使用std :: string作为键的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！