c++ push_back()和emplace_back()区别

c++ push_back()和emplace_back()区别

References

1. C++中push_back和emplace_back的区别
2. push_back v.s. emplace_back

一、源码分析

（1）push_back()定义

// stl_vector.h

template<typename _Tp, typename _Alloc = std::allocator<_Tp> >
    class vector : protected _Vector_base<_Tp, _Alloc>
{
      ...
      void
      push_back(const value_type& __x)
      {
	if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
	  {
	    _GLIBCXX_ASAN_ANNOTATE_GROW(1);
	    _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
				     __x);
	    ++this->_M_impl._M_finish;
	    _GLIBCXX_ASAN_ANNOTATE_GREW(1);
	  }
	else
	  _M_realloc_insert(end(), __x);
      }

#if __cplusplus >= 201103L
      void
      push_back(value_type&& __x)
      { emplace_back(std::move(__x)); }
}

1. 需传入对应类型的对象（隐式构造的情况除外）
2. C++11以及之后版本，传入右值需调用移动构造函数（若未定义，则调用拷贝构造函数）。

类的构造函数未添加explicit且只接受一个参数的情况，也可以传入单个参数进行构造。但涉及另一知识点：如果构造函数只接受一个实参，则它实际上定义了转换为此类类型的隐式转换机制，有时我们把这种构造函数称作转换构造函数（converting constructor）。

1. push_back传入参数不涉及类型推断，以下x为rvalue reference。

若没有一个特定的std::vector实例化, push_back是不存在的。实例化的类型完全决定了push_back的声明。不涉及类型推断。

template <class T, class Allocator = allocator<T> >
class vector {
public:
    ...
    void push_back(T&& x);       // fully specified parameter type => no type deduction;
    ...                          // && is rvalue reference
};

（2）emplace_back()定义

#if __cplusplus >= 201103L
  template<typename _Tp, typename _Alloc>
    template<typename... _Args>
#if __cplusplus > 201402L
      typename vector<_Tp, _Alloc>::reference
#else
      void
#endif
      vector<_Tp, _Alloc>::
      emplace_back(_Args&&... __args)
      {
	if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
	  {
	    _GLIBCXX_ASAN_ANNOTATE_GROW(1);
	    _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
				     std::forward<_Args>(__args)...);
	    ++this->_M_impl._M_finish;
	    _GLIBCXX_ASAN_ANNOTATE_GREW(1);
	  }
	else
	  _M_realloc_insert(end(), std::forward<_Args>(__args)...);
#if __cplusplus > 201402L
	return back();
#endif
      }
#endif

1. emplace_back既可以传入相应类型对象，还可以传入元素的类构造函数的参数列表进行原地构造。参数列表通过std::forward进行完美转发。
2. emplace_back的传入参数需要类型推断，以下Args为Universal Reference.emplace_back类型参数Args独立于vector模板类型参数T，所以Args类型在每次被调用的时候都必须被推断。

template <class T, class Allocator = allocator<T> >
class vector {
public:
    ...
    template <class... Args>
    void emplace_back(Args&&... args); // deduced parameter types => type deduction;
    ...                                // && is universal references
};

二、对比push_back和emplace_back

（1）定义测试类

class Foo{
private:
    int a;
public:
    Foo(int a): a(a){
        cout<<"Foo Constructor "<<a<<endl;
    }
    Foo(){
        cout<<"Default Foo Constructor"<<endl;
    }
    ~Foo() { cout << "Foo Destructor..." <<a<<endl; }
    Foo(const Foo& foo){
        cout<<"copy constructor"<<endl;
    }
    Foo(Foo&& foo){
        cout<<"move constructor"<<endl;
    }
};

（2）测试用例

1. push_back拷贝构造，emplace_back原地构造.

#include <iostream>
#include <vector>
using namespace std;
int main() {
    cout<<endl<<"push_back begin"<<endl<<endl;
    vector<Foo> vec1;
    Foo foo1(1);
    vec1.push_back(foo1);
    cout<<endl<<"push_back end"<<endl<<endl;

    cout<<endl<<"emplace_back begin"<<endl<<endl;
    vector<Foo> vec2;
    vec2.emplace_back(2);
    cout<<endl<<"emplace_back end"<<endl<<endl;
}

输出：

push_back begin

Foo Constructor 1
copy constructor //调用拷贝构造

push_back end

emplace_back begin

Foo Constructor 2  //vector上原地构造，无需拷贝，也无需移动

emplace_back end

Foo Destructor...2
Foo Destructor...1
Foo Destructor...0

1. 拷贝构造函数和移动构造函数均已定义，用push_back/emplace_back向容器中加入左值元素，均将调用拷贝构造函数拷贝对象到容器中。

#include <iostream>
#include <vector>
using namespace std;
int main() {
    cout<<endl<<"push_back begin"<<endl<<endl;
    vector<Foo> vec1;
    Foo foo1(1);
    vec1.push_back(foo1);
    cout<<endl<<"push_back end"<<endl<<endl;

    cout<<endl<<"emplace_back begin"<<endl<<endl;
    vector<Foo> vec2;
    Foo foo2(2);
    vec2.emplace_back(foo2);
    cout<<endl<<"emplace_back end"<<endl<<endl;
}

输出：

push_back begin

Foo Constructor 1
copy constructor

push_back end

emplace_back begin

Foo Constructor 2
copy constructor

emplace_back end

Foo Destructor...2 //左值离开作用域销毁
Foo Destructor...0
Foo Destructor...1 //左值离开作用域销毁
Foo Destructor...0

1. 拷贝构造函数和移动构造函数均定义，push_back/emplace_back向容器中加入右值元素， 那么均会调用移动构造函数在vector上进行构造。

• 加入显式构造的右值元素

vec1.push_back(Foo(1));
vec1.emplace_back(Foo(1));

• 通过std::move()转化而成的右值元素

Foo foo1;
vec1.push_back(std::move(foo1))
Foo foo2;
vec1.emplace_back(std::move(foo2));

• 测试

#include <iostream>
#include <vector>
using namespace std;
int main() {
    cout<<endl<<"push_back begin"<<endl<<endl;
    vector<Foo> vec1;
    vec1.push_back(Foo(1));
    cout<<endl<<"push_back end"<<endl<<endl;

    cout<<endl<<"emplace_back begin"<<endl<<endl;
    vector<Foo> vec2;
    vec2.emplace_back(Foo(2));
    cout<<endl<<"emplace_back end"<<endl<<endl;
}

输出：

push_back begin

Foo Constructor 1
move constructor
Foo Destructor...1  //右值立马销毁

push_back end

emplace_back begin

Foo Constructor 2
move constructor
Foo Destructor...2  //右值立马销毁

emplace_back end

Foo Destructor...0
Foo Destructor...0

1. 若仅定义拷贝构造函数，未定义移动构造函数，push_back/emplace_back向容器中加入右值元素， 那么均会调用拷贝构造函数构造对象在vector上。

#include <iostream>
#include <vector>
using namespace std;
class Foo{
private:
    int a;
public:
    Foo(int a): a(a){
        cout<<"Foo Constructor "<<a<<endl;
    }
    Foo(){
        cout<<"Default Foo Constructor"<<endl;
    }
    ~Foo() { cout << "Foo Destructor..." <<a<<endl; }
    Foo(const Foo& foo){
        cout<<"copy constructor"<<endl;
    }
    // Foo(const Foo&& foo){
    //     cout<<"move constructor"<<endl;
    // }
    // Foo(Foo&& foo)=delete;
};

int main() {
    cout<<endl<<"push_back begin"<<endl<<endl;
    vector<Foo> vec1;
    vec1.push_back(Foo(1));
    cout<<endl<<"push_back end"<<endl<<endl;

    cout<<endl<<"emplace_back begin"<<endl<<endl;
    vector<Foo> vec2;
    vec2.emplace_back(Foo(2));
    cout<<endl<<"emplace_back end"<<endl<<endl;
}

输出：

push_back begin

Foo Constructor 1
copy constructor
Foo Destructor...1  //右值立马销毁

push_back end

emplace_back begin

Foo Constructor 2
copy constructor
Foo Destructor...2  //右值立马销毁

emplace_back end

Foo Destructor...0
Foo Destructor...0