emplace_back和push_back区别，以及vector内存变化

参考链接

void* operator new(size_t size)//重载operator new
{
 	cout << "vector size to: " << size/sizeof(int) << endl;
	 return malloc(size);
}
class test {
public:
 	test() :num(0) { cout << "default construct of " <<0<< endl; };
 	test(int a) :num(a) { cout << "assign construct of " <<a<<endl; };
 	test(test&& t) :num(t.num) { cout << "move construct of " << t.num<< 	endl; };
 	test(const test& t) :num(t.num) { cout << "copy construct of " << t.num<< endl; };//参数前必须加const否则会报错
 //如果不定义，会调用默认的拷贝构造 copy bitwise
 	~test()
 	{
 	 cout << "deconstruct" << endl;
	 };
	private:
 	int num;
};
int main()
{
 	vector<test> a;
 	//https://blog.youkuaiyun.com/LiQian06336/article/details/99181398
	 cout << "test" << sizeof(test) << endl;
	 cout << "vector size in memory: " << sizeof(a) << endl;
 	cout << "vector capacity: " << a.capacity() << endl;
 	a.push_back(1);
 	cout << "vector capacity: " << a.capacity() << endl;
 	a.emplace_back(2);
 	/cout << "vector capacity: " << a.capacity() << endl;
 	a.push_back(test(3));
	 cout << "vector capacity: " << a.capacity() << endl;
 	a.emplace_back(test(4));
 	cout << "vector capacity: " << a.capacity() << endl;
}

运行结果，

vector size to: 2
test4
vector size in memory: 16//刚开始会申请内存，用于存放指针，见main函数中的链接
vector capacity: 0//正式开始
assign construct of 1//push_back(1),先构造临时对象
vector size to: 1//再申请内存
move construct of 1//再调用移动构造
deconstruct//销毁临时对象
vector capacity: 1
vector size to: 2//empalce_back，先申请内存，申请的是原来大小的两倍，因为vector要扩容，扩容不是原地扩，而是新找一个更大的地方
assign construct of 2//再在申请得到的新内存上原地构造
copy construct of 1//因为vector的容量由1变为2，需要将原来地方的1拷贝过来
deconstruct//将原地方的1析构
vector capacity: 2
assign construct of 3//同1一样，构造临时对象
vector size to: 3//申请内存，扩容
move construct of 3//将3先移动到新的内存地方上
copy construct of 1//将剩余的移动过来
copy construct of 2
deconstruct//将临时对象3和原来的1和2析构
deconstruct
deconstruct
vector capacity: 3
assign construct of 4
vector size to: 4
move construct of 4//同3
copy construct of 1
copy construct of 2
copy construct of 3
deconstruct
deconstruct
deconstruct
deconstruct
vector capacity: 4
deconstruct
deconstruct
deconstruct
deconstruct

结论：

• emplace_back和push_back区别在与以参数传入的时候
  ，emplace_back先申请内存，再原地构造，而push_back需要先构造临时对象。
• 对临时对象的处理上，都依赖与是否存在移动构造，若有，则都优先调用移动构造