push_back的效率惊人的低下，插入100个元素居然要分配(Reallocate)内存13次。

push_back elements # 1 2 3 4 5 7 10 14 20 29 43 64 95
Reallocate capacity # 1 2 3 4 6 9 13 19 28 42 63 94 141


1.测试代码

TEST(VECTOR, push_alloc)
{
using Container = vector<A>;
    Container container;
    for (int  i=0; i< 100; ++i){
        container.push_back(A("item"));
    }
}

2.调试步骤

在命令行中执行如下命令

windbg test.exe --gtest_filter=VECTOR.push_alloc

在windbg的命令中执行

0:000> r? $t0=0
0:000> x  test!std::vector<A,std::allocator<A> >::_Reallocate
00e9dfc0          test!std::vector<A,std::allocator<A> >::_Reallocate (unsigned int)
0:000> bp 00e9dfc0  ".printf \"count=%d num=%d\\n\", poi(@esp+4), @$t0;gc"
0:000> x test!std::vector<A,std::allocator<A> >::push_back
00e9f1d0          test!std::vector<A,std::allocator<A> >::push_back (class A *)
0:000> bp 00e9f1d0 "r $t0=@$t0+1;gc"
0:000> g
count=1 num=1
count=2 num=2
count=3 num=3
count=4 num=4
count=6 num=5
count=9 num=7
count=13 num=10
count=19 num=14
count=28 num=20
count=42 num=29
count=63 num=43
count=94 num=64
count=141 num=95

3.解决方案

调用vector::reserve()