What's the purpose of const parameters in a constructor?

Question

This is a program that uses classes, together with parameter constructors.

#include <string>

class Ball {

private:
    std:: string m_color;
    double m_radius;
public:

    Ball(const std::string& color, double radius)
    {
        m_color = color;
        m_radius = radius;
    }

    void print()
    {
        std::cout << "color: " << m_color << std::endl;
        std::cout << "radius: " << m_radius;
    }

};

int main()
{
    Ball blueTwenty{ "blue", 20.0 };
    blueTwenty.print();

    return 0;
}

Why do we have to add const before our first parameter? I see that the double-type parameter doesn't need one. Also, why do we need the first address of the parameter (I guess this is the reason for including &)? Could anyone explain the logic behind?

Sounds like you could use a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — NathanOliver, Apr 08 '20 at 17:40
The copy operator `std::string::operator=` expects a `const std::string&` which could otherwise be just used as a simple `std::string` if the string were to be modified anywhere inside the scope of constructor. This is not considered to be necessary but is a good habit to get into. — Ruks, Apr 08 '20 at 17:44
"why do we need the first address of the parameter" - Remember that C++ is a *context sensitive* language. `&` means different things in different contexts. It doesn't always mean "address of", here it means "reference" - it can also mean "bitwise and" in other contexts. — Jesper Juhl, Apr 08 '20 at 17:46

score 3 · Answer 1 · answered Apr 08 '20 at 17:41

It's not that it doesn't need one, but that the const was omitted. Changing radius won't impact the caller, but color, as a reference, could. Technically they should both be const unless they need to be modified.

const just says "I promise not to modify this argument" which the compiler can interpret a number of ways, including making optimizations in the code based on that.

It also avoids inadvertent mutation of arguments which can cause unpredictable side-effects.

What's the purpose of const parameters in a constructor?

1 Answers1