C++传值传参时拷贝构造与临时对象创建的疑问

给定代码 #

#include <iostream>
#include <vector>
#include <cstring>

class Person
{
public:
   Person(const char *i_Name = "Unknown", int i_Age = 0)
       : m_Name(copyFromTo(i_Name, m_Name)), m_Age(i_Age)
   {
      std::cout << "creating obj " << this << ".\n";
   }
   Person(const Person &other)
   {
      std::cout << "Copying from " << &other << " to " << this << ".\n";
      this->m_Name = copyFromTo(other.m_Name, this->m_Name);
      this->setAge(other.m_Age);
   }
   ~Person()
   {
      std::cout << "destroying obj " << this << ".\n";
      if (m_Name)
      {
         delete[] m_Name;
      }
   }
   void printDetails() { std::cout << "Hello, my name is " << m_Name << " and I am " << m_Age << " years old.\n"; }
   void setAge(int i_Age) { m_Age = i_Age; }
   int getAge() const { return m_Age; }

private:
   char *copyFromTo(const char *i_Source, char *i_Destination)
   {
      int sourceSize = strlen(i_Source);
      i_Destination = new char[sourceSize + 1];
      memcpy(i_Destination, i_Source, sourceSize);
      i_Destination[sourceSize] = '\0';
      return i_Destination;
   }
   char *m_Name;
   int m_Age;
};

Person birthday(Person person)
{
   std::cout << "person's address inside the function " << &person << ".\n";
   person.setAge(person.getAge() + 1);
   return person;
}

int main()
{
   Person person = {"New Person", 24};
   birthday(person);
   std::cout << "END OF PROGRAM.\n";
}

运行输出 #

creating obj 0x61fea8.
Copying from 0x61fea8 to 0x61feb8.
person's address inside the function 0x61feb8.
Copying from 0x61feb8 to 0x61feb0. // [临时对象创建]
destroying obj 0x61feb0. // [临时对象销毁]
destroying obj 0x61feb8.
END OF PROGRAM.
destroying obj 0x61fea8.

问题 #

调用birthday函数时，因参数按值传递创建了局部person对象；但函数结束返回时，又调用拷贝构造函数创建了一个临时对象，且该临时对象在局部person对象销毁前就被销毁。请问为何需要这个临时对象？它的实际作用是什么？

解答 #

这个临时对象是C++按值返回机制的必然产物：函数内部的局部person对象（地址0x61feb8）属于函数栈帧，函数执行完毕后栈帧会被销毁，局部对象的内存也会被释放，根本没法直接把它的内容交给调用者。

所以当birthday返回person时，必须先把这个局部对象的内容拷贝到临时对象里（地址0x61feb0）——这个临时对象位于调用者的上下文内存区域，生命周期由调用方管理。

在你的代码里，这个临时对象创建后没有被任何变量接收，所以它刚完成“中转返回值”的使命就被销毁了，之后才轮到函数的局部对象被销毁。

总结来说，它的核心作用就是作为返回值的中转载体：把即将消失的局部对象的内容传递到调用者的上下文里，哪怕调用者没接住这个返回值，这个中转流程也得执行（除非编译器触发了返回值优化RVO来省略临时对象）。

内容的提问来源于stack exchange，提问作者Yarin0600