[C++知识库]【C++】emplace_back vs push_back

C++11 引入了 vector::emplace_back()，此前 vector 只能调用 push_back 来添加元素。一个简单的示例来对比两者区别：

#include <iostream>
#include <vector>

using namespace std;

struct A {
    A() { cout << "Default constructor: " << val << endl; }
    A(int v) : val(v) { cout << "Construct with val: " << val << endl; }
    A(const A& a) : val(a.val + 10) { cout << "Copy constructed: " << val << endl; }
    A(A&& a) : val(a.val + 100) { cout << "Move constructed: " << val << endl; }
    ~A() { cout << "Destructed: " << val << endl; }
    int val { 0 };
};

int main()
{
    A a(1), b(2);
    vector<A> vec;
    vec.reserve(4);
    cout << endl;

    cout << "push_back existing item:\n";
    vec.push_back(a);
    cout << endl;

    cout << "emplace_back existing item:\n";
    vec.emplace_back(b);
    cout << endl;

    cout << "push_back non-existing item:\n";
    vec.push_back(3);
    cout << endl;

    cout << "emplace_back non-existing item:\n";
    vec.emplace_back(4);
    cout << endl;
}

执行结果：

Construct with val: 1
Construct with val: 2

push_back existing item:
Copy constructed: 11

emplace_back existing item:
Copy constructed: 12

push_back non-existing item:
Construct with val: 3
Move constructed: 103
Destructed: 3

emplace_back non-existing item:
Construct with val: 4

Destructed: 11
Destructed: 12
Destructed: 103
Destructed: 4
Destructed: 2
Destructed: 1

可以看到，当被添加的元素已经存在时，push_back 与 emplace_back 是一样的。当元素事先不存在时，emplace_back 是直接在原地构造一个对象，而 push_back 会先构造一个临时对象，再将其移动到 vector 中，然后析构掉临时对象。基于这样的表现，我们在平时大部分情况下都应当用 emplace_back 替代 push_back。

那是不是 push_back 就一无是处了呢？

Why would I ever use push_back instead of emplace_back? 中提到，当使用隐式构造函数时，push_back 能够提供更安全的编译检查，比如：

vector<unique_ptr<int>> v;
int a;
v.emplace_back(std::addressof(a)); // compiles
v.push_back(std::addressof(a)); // fails to compile

vector<int8_t> vec;
int a = 1000;
vec.emplace_back(a); // no warning
vec.push_back(a); // -Wconversion warning