回答編集履歴
1
string の構造を推定
answer
CHANGED
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@@ -28,4 +28,58 @@
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28
28
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cout << endl;
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29
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}
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30
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}
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31
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```
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32
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**追記**
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33
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最近の gcc(64ビット) では、sizeof(std::string) は 32 です。
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34
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std::string のソースは見ていませんが、stringオブジェクトのダンプを見て、
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35
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次のように妄想してみました。
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36
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```C++
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37
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#include <iostream>
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38
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#include <string>
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39
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using namespace std;
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40
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41
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struct String {
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char *data;
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43
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uint64_t size;
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union {
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45
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char here[16];
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uint64_t capacity;
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};
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};
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50
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void dump(string &s)
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51
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{
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cout << "size() = " << s.size() << ", capacity() = " << s.capacity()
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53
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<< ", s.data() = [" << s.data() << "]\n";
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54
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String *p = reinterpret_cast<String *>(&s);
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55
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56
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char *data;
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57
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uint64_t capacity;
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58
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if (p->size < 16) data = p->here, capacity = 15;
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59
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else data = p->data, capacity = p->capacity;
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60
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cout << "String: size = " << p->size << ", capacity = " << capacity
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61
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<< ", data = [" << data << "]\n\n";
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62
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}
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63
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64
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int main()
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65
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{
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66
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string s;
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67
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dump(s);
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68
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s = "abc";
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69
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dump(s);
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70
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s = "abcdefghijklmnopqrstuvwxyz";
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dump(s);
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72
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}
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73
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```
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74
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実行結果
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75
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```
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76
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size() = 0, capacity() = 15, s.data() = []
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77
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String: size = 0, capacity = 15, data = []
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78
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79
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size() = 3, capacity() = 15, s.data() = [abc]
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80
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String: size = 3, capacity = 15, data = [abc]
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81
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82
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size() = 26, capacity() = 30, s.data() = [abcdefghijklmnopqrstuvwxyz]
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83
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String: size = 26, capacity = 30, data = [abcdefghijklmnopqrstuvwxyz]
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84
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31
85
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```
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