更改默认的复制构造函数C ++

Question

I am having problems understanding how to override the default copy constructor in C++. I am receiving no compilation errors. The previous examples show to me our of the pattern below. Below are listed excerpts from the files HashTable.cpp and Hashtable.h .

Hashtable.h

HashTable& operator=(const HashTable& other);`

HashTable.cpp

const HashTable& HashTable::operator=(const HashTable& other) {
    std::cout << "EQUAL OPERATOR METHOD" << std::endl;
    return *this;
}

main.cpp

HashTable ht1 {9};
HashTable ht2 { ht1 };

Although when I compile, it appears as if the copy constructor is not called. To clarify, I am trying to copy one variable to the other.

It is worth noting that I am coding in c++11 on Ubuntu 14.04. As coding c++ in Ubuntu has already had many hangups for me, I am uncertain if this is c++ or ubuntu problem. I have spent quite some time trying to figure out what is going on here so please do not down vote.

Answer 1

The code you wrote above is the overriding of copy assignment operator , but, according to your main.cpp , seems like you need the copy constructor (don't be scared by the amount of text in these descriptions, it's really easy to understand). Try the following code:

HashTable.h

class HashTable
{
private:
    // private members declaration...
public:
    // public members declaration...
    HashTable(const HashTable& other);
}

HashTable.cpp

// Copy constructor implementation
HashTable::Hashtable(const HashTable& other){
    // implement your own copy constructor
    std::cout << "OVERRIDED COPY CONSTRUCTOR METHOD" << std::endl;
    // This is the constructor, so don't have to return anything
}

main.cpp

HashTable ht2(ht1);

PS: Not sure about HashTable ht2 {ht1} (using the symbols { and } ). Seems like it's C++14 feature, according to the comment of MM .

Answer 2

First of all, the sample code in your question is a copy assignment. The difference between the copy constructor and the copy assignment is that, the copy construction can only be called when initializing an object. After an object is initialized and you want to pass another initialized object to it, the copy assignment is rather called. So in the main function since ht1 was passed to ht2 while initializing it, it will rather call the copy constructor. But in your code, you defined the copy assignment and not the copy constructor. check out copy assignment c++ for more details about the difference between the copy assignment and the copy constructor.

Answer 3

HashTable::operator=(const HashTable& other) is assignment opperator. Copy constructor should be written as HashTable::HashTable(const HashTable& other).

" HashTable ht2 { ht1 } " is not invoking copy-constructor, it's actually invoking initializer_list: " HashTable(initializer_list< HashTable>) "

To invoke copy constructor you should write HashTable hash(anotherHashTable) in your main.cpp.