What does const K& k = K() mean in this constructor function?

Question

I am having difficulty understanding the syntax of this code fragment

What is the purpose of "const K& k = K(), const V& v = V()"?

template<typename K, typename V>
class Entry{

    public:
        typedef K Key;
        typedef V Value;
    public:
        Entry(const K& k = K(), const V& v = V()) : _key(k), _value(v) {}

    ....
        ....
            ....
    private:
        K _key;
        V _value;
};

Thank you

Do you know what a reference is? Do you know what a default function parameter is? If not you should get a [good C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — NathanOliver, Sep 20 '17 at 19:49
it's c++ template syntax. K and V can be any type of class or structure. — jesuisgenial, Sep 20 '17 at 20:13
this is injecting instance of K and V into class's private property — jesuisgenial, Sep 20 '17 at 20:14
@jesuisgenial c++ class members are not called "properties" and calling default parameters to a constructor "injection" may lead to confusion too imho — 463035818_is_not_an_ai, Sep 20 '17 at 20:23
you guys are right. i'm a bit confused too. thanks for revise — jesuisgenial, Sep 20 '17 at 21:01

Remy Lebeau · Answer 1 · 2017-09-20T20:53:02.790

K and V are template parameters, they can be any data type the user of Entry wants.

k and v are input parameters of the Entry constructor. They are being passed in by const reference, and they have default values specified, where K() default-constructs a temp object of type K, and V() default-constructs a temp object of type V. If the user does not provide explicit input values when creating an Entry object instance, the defaults are used instead.

For example:

Entry<int, int> e1(1, 2);
// K = int, k = 1
// V = int, v = 2

Entry<int, int> e2(1);
// aka Entry<int, int> e2(1, int())
// K = int, k = 1
// V = int, v = 0

Entry<int, int> e3;
// aka Entry<int, int> e3(int(), int())
// K = int, k = 0
// V = int, v = 0

Entry<std::string, std::string> e4("key", "value");
// K = std::string, k = "key"
// V = std::string, v = "value"

Entry<std::string, std::string> e5("key");
// aka Entry<std::string, std::string> e4("key", std::string())
// K = std::string, k = "key"
// V = std::string, v = ""

Entry<std::string, std::string> e6;
// aka Entry<std::string, std::string> e4(std::string(), std::string())
// K = std::string, k = ""
// V = std::string, v = ""

It might not hurt to talk about what the syntax `K()` and `V()` mean, since I think the OP doesn't know you can do that. — templatetypedef, Sep 20 '17 at 20:34

What does const K& k = K() mean in this constructor function?

1 Answers1