Vector 增长变化

最新推荐文章于 2024-09-02 14:06:19 发布

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最新推荐文章于 2024-09-02 14:06:19 发布

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VS 2019 亲测，绝对详细。

    std::vector<char> v6;
    auto v6_capacity = v6.capacity();
    std::cout << "[V6]:" << std::endl;
    std::cout << "Size:" << v6.size() << "  MaxSize" << v6.max_size() << std::endl;
    std::cout << "Capacity:" << v6.capacity() << std::endl;
    for (int i = 0; i < 1000; i++)
    {
        v6.push_back('1'); //capacity 1, size 1, add allocator 1
        if (v6.capacity() - v6_capacity)
        {
            std::cout << "Size:" << v6.size() << std::endl;
            std::cout << "Capacity:" << v6.capacity() << std::endl;
            std::cout << "add allocated apacity:" << v6.capacity() - v6_capacity << std::endl << std::endl;
        }
        v6_capacity = v6.capacity();
    }

[V6]:
Size:0  MaxSize2147483647
Capacity:0
Size:1
Capacity:1
add allocated apacity:1

Size:2
Capacity:2
add allocated apacity:1

Size:3
Capacity:3
add allocated apacity:1

Size:4
Capacity:4
add allocated apacity:1

Size:5
Capacity:6
add allocated apacity:2

Size:7
Capacity:9
add allocated apacity:3

Size:10
Capacity:13
add allocated apacity:4

Size:14
Capacity:19
add allocated apacity:6

Size:20
Capacity:28
add allocated apacity:9

Size:29
Capacity:42
add allocated apacity:14

Size:43
Capacity:63
add allocated apacity:21

Size:64
Capacity:94
add allocated apacity:31

Size:95
Capacity:141
add allocated apacity:47

Size:142
Capacity:211
add allocated apacity:70

Size:212
Capacity:316
add allocated apacity:105

Size:317
Capacity:474
add allocated apacity:158

Size:475
Capacity:711
add allocated apacity:237

Size:712
Capacity:1066
add allocated apacity:355

刚刚做了一个实验，有人说是增长50%。但是！！！重点来了。具体多少可以自己算，毕竟标准库没有规定，VS2019的capacity增长结果来看：
+1（0.5进位多计算0.5）A
+1（0.5舍去不进位补回0.5）B
+1（0.5进位多计算0.5）A
+1（0.5舍去不进位补回0.5）B
+2(1整除) 0
+3(1.5进位多计算0.5) A
+4(整除)
+6(整除)
+9(不进位补回0.5) B
+14(整除)
+21(进位多计算0.5) A
+31(不进位补回0.5) B
+47(进位多计算0.5) A
+70(整除)
+105(不进位补回0.5) B
+158(整除)
+237(进位多计算0.5) A
+355(不进位补回0.5) B
. 如果前一次4舍5入进位后，下次就4舍5不入，不进位。

为什么要在意这个增长速度呢？

当Vector里面放的东西足够多，足够大时。就会影响效率。
因为Vector 每次增长，会创建一个新的 Vector也就是增长大小以后的Vector，然后会把所有东西都copy过去。然后清除原来的。这样就会造成很多很多次copy，如果是对象会进入多次拷贝构造。
当然如果自定义类型是支持移动构造函数的化，vector的copy会优先使用移动构造来交换。