push_back讲解

push_back函数在C++中的vector头文件中，在vector类中作用为在vector尾部加入一个数据。string中也有这个函数，作用是字符串之后插入一个字符。和insert相比push_back,insert 放到指定位置，push_back放到最后一位。

push_back是stl里面容器的方法。
比如vector que;
que.push_back(‘a’);
que.push_back(‘b’);
push_back在作用是在容器里添加相应的元素。
que.push_back((char)(dep+1+’0’));的意思是向容器中加入一个（dep+1+’0’）的字符

例如:
当主函数只有一个push_back时：
int main()
{
vector sb;
HP a;
sb.push_back(a);
while(1);
}
由于原来的vector为空，所以只是分配新的空间，然后拷贝push_back的元素。所以结果如下：
gouzao //这是 语句 HP a; 构造a产生
kaobei //这是 语句 sb.push_back(a) 拷贝新元素产生的。
当主函数有两个push_back时：
int main()
{
vector sb;
HP a;
sb.push_back(a);
sb.push_back(a);
while(1);
}
注意执行第二个push_back时，首先另外分配两个元素的空间，并将原来的vector元素拷过来，后拷贝新push的元素，最后析构原来的元素。故结果如下 ：
gouzao //这是 语句 HP a; 构造a产生
kaobei //这是 第一个sb.push_back(a) 拷贝新元素产生的。
kaobei //下面这两个是由第二个sb.push_back(a)产生的，它先先将原来的vector元素拷过来，后拷贝新push的元素
kaobei
xigou //只是析构原来的vector元素产生的。
当主函数有三个push_back时：一样分析，
int main()
{
vector sb;
HP a;//构造
sb.push_back(a); //一个拷贝
sb.push_back(a);//两个拷贝，一个析构
sb.push_back(a);//三个拷贝，两个析构
while(1);
}
结果如下：
gouzao
kaobei
kaobei
kaobei
xigou
kaobei
kaobei
kaobei
xigou
xigou

下面是看别人博客看到的例子：
vector有预存的内存（capacity），如果存入的元素大于了capacity，就重新分配一个比原来capacity大两倍的内存。

#include <iostream>#include <vector>using namespace std;class test {public:    string a;    test(string b) : a(b) {        cout << "new" << " " << a << endl;    }    ~test() {        cout << "deleted " << a << endl;    }};int main(int argc, const char * argv[]) {    vector<test> array;    string a = "A";    array.push_back(test("a"));    cout << array.capacity() << endl;    array.push_back(test("b"));    cout << array.capacity() << endl;    array.push_back(test("c"));    cout << array.capacity() << endl;    array.push_back(test("d"));    cout << array.capacity() << endl;    array.push_back(test("f"));    cout << array.capacity() << endl;    return 0;}

输出：

new adeleted a1new bdeleted adeleted b2new cdeleted bdeleted adeleted c4new ddeleted d4new fdeleted ddeleted cdeleted bdeleted adeleted f8deleted fdeleted ddeleted cdeleted bdeleted aProgram ended with exit code: 0

结合一起看：

#include <iostream>#include <vector>using namespace std;struct test{    test(char c) :a(c)    {        cout << "New " << a << endl;    }    ~test()    {        cout << "delete " << a << endl;    }    char a;};int main(){    vector<test> t;    for (char c = 'a'; c < 'z' + 1; ++c) {        t.push_back(c);        cout << t.capacity() << endl;    }    system("pause");}

输出：

New adelete a1New bdelete adelete b2New cdelete bdelete adelete c4New ddelete d4New edelete ddelete cdelete bdelete adelete e8New fdelete f8New gdelete g8New hdelete h8New idelete hdelete gdelete fdelete edelete ddelete cdelete bdelete adelete i16New jdelete j16New kdelete k16New ldelete l16New mdelete m16New ndelete n16New odelete o16New pdelete p16New qdelete pdelete odelete ndelete mdelete ldelete kdelete jdelete idelete hdelete gdelete fdelete edelete ddelete cdelete bdelete adelete q32New rdelete r32New sdelete s32New tdelete t32New udelete u32New vdelete v32New wdelete w32New xdelete x32New ydelete y32New zdelete z32delete zdelete ydelete xdelete wdelete vdelete udelete tdelete sdelete rdelete qdelete pdelete odelete ndelete mdelete ldelete kdelete jdelete idelete hdelete gdelete fdelete edelete ddelete cdelete bdelete aProgram ended with exit code: 0