Compiler can't see argument

Question

I am new to C++ and want to play around with classes.

My code

In my world there are heros and swords. Heros carry swords. This shouldn't be too hard.

// Defining swords                                                                                                                                 
class Sword
{
  // The most important thing about a sword is its length.                                                                                         
  int lenght;
public:
  // only constructor and destructor                                                                                                               
  Sword(int swordlength){
    lenght = swordlength;
  };
  ~Sword(){};
};

// defining heros (as people with magic swords)                                                                                                    
class Hero
{
  Sword magic_sword;
public:
  // each hero gets a standard sword                                                                                                               
  Hero(){
    int meters = 2;
    magic_sword = Sword(meters);
  };
  ~Hero(){};
};

int main(){
  return 0;
}

What the compiler thinks about it

When I compile this code (g++ hero.cpp) I get an error:

 In constructor 'Hero::Hero()':
20:9: error: no matching function for call to 'Sword::Sword()'
20:9: note: candidates are:
8:3: note: Sword::Sword(int)
8:3: note:   candidate expects 1 argument, 0 provided
2:7: note: constexpr Sword::Sword(const Sword&)
2:7: note:   candidate expects 1 argument, 0 provided

What I think the problem is

Compiling the code with clang++ also fails, but the error message is not as explicit, so I won't post it here.

It seems that calling the constructor Sword(meters) failed because I provided 0 instead of 1 argument. But I clearly gave it an argument (meters), so I guess I am missunderstanding something here.

What is my mistake and what can I do about it?

Answer 1

Your Hero always has a Sword.

That means that, the moment the class is instantiated, before the Hero::Hero constructor starts running, it already has to have a "default" Sword. The compiler is complaining that it doesn't know how to make such a "default" Sword.

Now, there's multiple ways you can address this:

Using initialization lists

In the definition of a constructor, you can specify how your object's members should be initialized (before the actual constructor runs). This is different from assigning them a new value in the constructor's body (which happens in your example). To do this, specify a colon (:) after the constructor's signature followed by the member field initializers, like this:

Hero() : magic_sword(2)
{
  // do other stuff to set up your hero
}

This will cause the magic_sword to be initialized using the Sword::Sword(int) constructor right away, which means you don't need to have a default Sword::Sword() constructor.

Read more: Understanding Initialization Lists in C++

Using pointers

Do you really want your Sword to belong exclusively to your Hero? Can your Hero maybe drop a Sword, and another Hero picks it up?

In this case, you probably don't want your Hero to have a Sword as a member - instead, you'd want a pointer to a Sword that he's currently holding - which can then maybe be nullptr if he's, for example, not holding a sword!

Answer 2

Your Sword class does not have a constructor with no parameters. At the beginning of Hero() (before the opening {), the members that are not specified in the initializer list get default constructed.

Try this:

Hero() : magic_sword(2) {
};

Or you can add another constructor to Sword that takes no parameters.

Sword(){
    lenght = -1;
};