Parameterized constructor

Learn to initialize objects with parameterized constructors.

Parameterized constructor

A constructor can also take parameters. This lets you give an initial value from outside when the object is created. A parameterized constructor is written with a parameter list in parentheses after the class name.

Example: class Score { public: Score(int s) { points = s; } int points; };. Usage: Score score(100);. This line both creates the object and sets points to 100. Once a parameterized constructor is defined, the default constructor is no longer generated automatically; you must write one yourself if needed.

Direct initialization

There are two ways to create an object with a parameterized constructor. The classic way uses parentheses: Score score(100);. Modern C++ also allows braces: Score score{100};. Both call the same constructor.

The brace form has the advantage of rejecting narrowing conversions. Score score{3.5}; gives a compile error if Score(int) is defined, because 3.5 narrows from double to int. The parenthesis form would silently convert and lose data. That is why modern C++ prefers the brace form.

Exercise Add a parameterized constructor to the Score class

Define a Score class. Under public: add the constructor Score(int s) { points = s; } and the int points; member variable. In main create an instance with Score score(100); and print points with std::cout.

Show example solution

#include <iostream>

class Score {
public:
    Score(int s) {
        points = s;
    }
    int points;
};

int main() {
    Score score(100);
    std::cout << score.points;
    return 0;
}

Test yourself

Complete the parameterized constructor call
```
Score score____;
```
Show answer
```
Score score(100);
```
If only Score(int) is defined in Score, what happens with Score s;?
```
class Score { public: Score(int s); };
```
- Works, default constructor is called
- Works, s is zeroed
- Gives a compile error
- Works, s gets a garbage value
Show answer

Gives a compile error — With a parameterized constructor, no default constructor is generated.
Match each call to its result

Score s(100);

100, parentheses
Score s{100};

100, braces
Score s{3.5};

compile error
Score s;

default constructor

Next lesson: Member initializer list

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies