C++ emplace_back 和 push_back 的区别
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2022-03-21 16:33:18
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emplace_back和push_back都是向容器内添加数据.
对于在容器中添加类的对象时, 相比于push_back,emplace_back可以避免额外类的复制和移动操作.
"emplace_back avoids the extra copy or move operation required when using push_back."
为了证实上述论断,我们自定义一个类,并在普通构造函数、拷贝构造函数、移动构造函数中打印相应描述:
#include <vector>
#include <string>
#include "time_interval.h"
class Foo {
public:
Foo(std::string str) : name(str) {
std::cout << "constructor" << std::endl;
}
Foo(const Foo& f) : name(f.name) {
std::cout << "copy constructor" << std::endl;
}
Foo(Foo&& f) : name(std::move(f.name)){
std::cout << "move constructor" << std::endl;
}
private:
std::string name;
};
int main() {
std::vector<Foo> v;
int count = 10000000;
v.reserve(count); //预分配十万大小,排除掉分配内存的时间
{
Foo temp("ceshi");
v.push_back(temp);// push_back(const T&),参数是左值引用
//打印结果:
//constructor
//copy constructor
}
v.clear();
{
Foo temp("ceshi");
v.push_back(std::move(temp));// push_back(T &&), 参数是右值引用
//打印结果:
//constructor
//move constructor
}
v.clear();
{
v.push_back(Foo("ceshi"));// push_back(T &&), 参数是右值引用
//打印结果:
//constructor
//move constructor
}
v.clear();
{
std::string temp = "ceshi";
TIME_INTERVAL_SCOPE("push_back(string):");
v.push_back(temp);// push_back(T &&), 参数是右值引用
//打印结果:
//constructor
//move constructor
}
v.clear();
{
std::string temp = "ceshi";
v.emplace_back(temp);// 只有一次构造函数,不调用拷贝构造函数,速度最快
//打印结果:
//constructor
}
}