返回对象时编译器会做什么

Question

I have the following class called DATA.

enum DATATYPE{DATATYPE_CONSTANT=0, DATATYPE_NUMBER,DATATYPE_STRING, DATATYPE_MATRIX, DATATYPE_OBJECT};

struct DATA //A Data container for the variable
{
    DATA(DATATYPE type,int row=0,int col=0)
    {
        m_str=0;m_number=0;

        m_DataType=type;

        if(type==DATATYPE_NUMBER) m_number=new double;
        if(type==DATATYPE_STRING) m_str=new string("");

        cout<<"In constructor"<<endl;
        //if(type==DATATYPE_MATRIX) m_matrix= new MatrixXd(row,col);
    }
    ~DATA()
    {
        if(m_str) m_str->clear();
        if(m_number) {delete m_number; m_number=0;}

        std::cout<<"In Destructor"<<std::endl;
        //if(m_matrix) {delete m_matrix; m_matrix=0;}
    }

    DATA(const DATA& other)
    {
        m_number=other.m_number;
        m_str=other.m_str;
        m_DataType=other.m_DataType;

        cout<<"In copy constructor"<<endl;
    }

    DATA& operator=(const DATA& other)
    {
        m_number=other.m_number;
        m_str=other.m_str;
        m_DataType=other.m_DataType;

        cout<<"In operator="<<endl;

        return *this;
    }

    DATATYPE GetType()
    {
        return m_DataType;
    }

    double* GetNumber()
    {
        return m_number;
    }

    void SetNumber(const double& val){*m_number=val;}


    string* GetString()
    {
        return m_str;
    }

private:
    DATATYPE m_DataType;
    string* m_str;
     //MatrixXd* m_matrix;
    double* m_number;
};

And I have the following test:

DATA GetData();

int main()
{
    cout<<"Before GetData call"<<endl;
    DATA dat=GetData();
    //DATA dat2=dat;
    cout<<*(dat.GetNumber())<<endl;
    cout<<"After Get Data call"<<endl;

    cout << "Exiting main" << endl;
    return 0;
}


DATA GetData()
{
    cout<<"In Get Data"<<endl;
    DATA ret(DATATYPE_NUMBER);
    double d=5;
    ret.SetNumber(d);
    cout<<"Exiting GetData"<<endl;
    return ret;
}

After running the test the output is:

Before GetData call

In Get Data

In constructor

Exiting GetData

5

After Get Data call

Exiting main

In Destructor

I have the following questions:

1. When I call DATA dat=GetData(); it neither calls constructor, copy constructor nor equal operator. How is dat object constructed. Also what exactly compiler do when returning from GetData ?

2. For the DATA structure, or in general aggregate data types, is it always a good idea to initialize member variables with new ? What happens to member variables when I initialize say DATA *d=new DATA(DATATYPE_NUMBER) ? Is it getting more error prone for memory leaks?

Answer 1

Question 1

When I call DATA dat=GetData(); it neither calls constructor, copy constructor nor equal operator. How is dat object constructed. Also what exactly compiler do when returning from GetData ?

Answer That is a result of something called return value optimization (RVO) . You can read more about them here .

In g++, you can disable RVO by using the flag -fno-elide-constructors . If you do that with your code, you will see messages from the copy constructor.

Question 2

For the DATA structure, or in general aggregate data types, is it always a good idea to initialize member variables with new? What happens to member variables when I initialize say DATA *d=new DATA(DATATYPE_NUMBER) ? Is it getting more error prone for memory leaks?

Answer

There are three question in that.

Answer 2.1

The answer to that question is "depends on your application". For some, having objects as member data makes sense while for others, having pointers to objects make sense. When you use pointers to objects, you have to follow the Rule of Three , which has become the Rule of Five in C++11 .

Answer 2.2

The member variables are initialized just as they would be had you used:

Data d = DATA(DATATYPE_NUMBER);

Answer 2.3

Using dynamic memory has benefits, but it also has its down sides. Any time you use dynamic memory allocation, you are entering into more error prone code. You have to worry about its potential bad side effects:

1. Dangling pointers.
2. Lost pointers.
3. Accessing memory beyond what was allocated.

Answer 2

When I call DATA dat=GetData(); it neither calls constructor, copy constructor nor equal operator. How is dat object constructed. Also what exactly compiler do when returning from GetData?

equal operator : Even if it may look like you are assigning, that is not the case. The syntax T t = u; does not contain any assignment, but rather copy construction.

constructor : The constructor is called inside the function, then a copy is returned and that copy is in turn copied over the destination object. Except it isn't. The language allows for copy-elision which means that the compiler is allowed to remove the copies by placing the three objects ( ret inside the function, returned object and dat inside main) in the same memory location.

I am not sure how much the exact details matter or will help, but in most ABIs (in all I know) a function returning by value an object is transformed by the compiler into a function that takes a pointer to the location where the object will live.

T f(int x) {
   T tmp(x);
   return tmp;
}
int main() {
   T t = f(1);
}

Is transformed into:

void f(void *__ret, int x) {
   new (__ret) T(x); // constructor call
   return;                               // return does not *copy*
}
int main() {
   [[uninitialized]] T t;                // space is reserved, no construction
   f(&t, 1);
}

That is literally how copy-elision functions, f created the tmp object on top of the returned value (the argument) and the caller in main placed the t and the return value of f over the same memory location.

For the DATA structure, or in general aggregate data types, is it always a good idea to initialize member variables with new? What happens to member variables when I initialize say DATA *d=new DATA(DATATYPE_NUMBER) ? Is it getting more error prone for memory leaks?

You tell me. But before you answer, note that your DATA type has a memory leak.

Answer 3

Make your constructors explicit and remove the default parameters on DATA(enum,int,int). When GetData() constructs the object with a single parameter you are most likely getting a compiler generated default constructor.

You can save memory by putting m_str & m_number in a union rather than separate member variables:

DATA {
...

union U {
   string* m_string;
   double* m_number;
}
    DATATYPE m_DataType;
    U m_data;
}

Using pointers for members is both fine and common when the members are large or can be constructed late for performance. But you do have to take additional care to avoid memory leaks. OTOH, if the members are small, the convenience of a regular member variable is frequently best.