Compare two objects in C++

This post will discuss how to compare two objects in C++.

We can determine whether two objects are the same by implementing a comparison operator== for the class. For instance, the following code compares two objects of class Node based on the value of x and y field.

#include <iostream>

class Node {

public:

int x, y;

Node(int x, int y): x(x), y(y) {}

bool operator==(const Node& rhs) const;

};

bool Node::operator==(const Node& rhs) const {

return x == rhs.x and y == rhs.y;

}

int main()

{

Node node1(1, 1);

Node node2(1, 1);

bool isEqual = (node1 == node2);

std::cout << std::boolalpha << isEqual << std::endl;

return 0;

}

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The object comparison should be done on all the fields that are critical for determining its equality. Starting with C++11, we can construct a std::tie to easily match against multiple fields, as shown below:

#include <iostream>

#include <tuple>

class Node {

public:

int x, y;

Node(int x, int y): x(x), y(y) {}

bool operator==(const Node& rhs) const;

};

bool Node::operator==(const Node& rhs) const {

return std::tie(x, y) == std::tie(rhs.x, rhs.y);

}

int main()

{

Node node1(1, 1);

Node node2(1, 1);

bool isEqual = (node1 == node2);

std::cout << std::boolalpha << isEqual << std::endl;

return 0;

}

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We can also implement the comparison operator!= to determine whether two objects of a class are not the same. This is demonstrated below:

#include <iostream>

class Node {

public:

int x, y;

Node(int x, int y): x(x), y(y) {}

bool operator==(Node const &rhs) const;

bool operator!=(Node const &rhs) const;

};

bool Node::operator==(Node const &rhs) const {

return x == rhs.x and y == rhs.y;

}

bool Node::operator!=(Node const &rhs) const {

return x != rhs.x or y != rhs.y;

}

int main()

{

Node node1(1, 1);

Node node2(1, 1);

bool isNotEqual = node1 != node2;

std::cout << std::boolalpha << isNotEqual << std::endl; // false

return 0;

}

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Note that the equality operators operator== and operator!= need not be members of the class. It can be implemented as follows:

#include <iostream>

class Node {

public:

int x, y;

Node(int x, int y): x(x), y(y) {}

};

bool operator==(Node const &lhs, Node const &rhs) {

return lhs.x == rhs.x and lhs.y == rhs.y;

}

bool operator!=(Node const &lhs, Node const &rhs) {

return lhs.x != rhs.x or lhs.y != rhs.y;

}

int main()

{

Node node1(1, 1);

Node node2(1, 1);

bool isEqual = (node1 == node2);

std::cout << std::boolalpha << isEqual << std::endl; // true

return 0;

}

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That’s all about comparing two objects in C++.

Compare two objects in C++

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