该类不存在默认构造函数，但未传递默认值

Question

My Thing class is derived from Entity class which has the following constructor

    Entity::Entity(string sNames, double xcord, double ycord)
    :m_sName(sNames), m_dX(xcord),m_dY(ycord){}

Thing's constructor is

    Thing::Thing(string sName, double xcord, double ycord):
    Entity(sName, xcord, ycord),
    m_iHealth(100),m_Weapon(Weapon("Fists", false, 10.0, 5, 1.0, xcord, ycord)){}

The problem is I get error "no appropriate default constructor available" on my Thing constructor. What is the problem I specified to use my Entity constructor not a default. To make the problem even more confusing for me I have another class derieved from Entity that works

    Weapon::Weapon(string sName, bool iMagical, double dRange, int iDamage,double
    dRadius, double dSpawnX, double dSpawnY):
    Entity(sName, dSpawnX, dSpawnY), m_bMagical(iMagical), m_dRange(dRange), m_iDamage(iDamage),
    m_dRadius(dRadius)
{
}

That runs without error but it appears to be the same exact thing as my Thing constructor with more variables. I'm sure I am missing something small but I have been stumped for a while.

You were correct there was some leftover code that didn't get commented out. It seems strange that and error in member variable deceleration shows up in constructor, but thanks anyway. It's always the simple things hat get me.

Answer 1

Perhaps your Thing does not, in fact, have a default constructor and one is required. A program like this might generate an error like you saw.

#include <string>
using std::string;

struct Entity {
  std::string m_sName;
  double m_dX, m_dY;
  Entity(std::string, double, double);
};
struct Weapon : Entity {
  bool m_bMagical;
  double m_dRange;
  int m_iDamage;
  double m_dRadius;
  Weapon(std::string, bool, double, int, double, double, double);
};
struct Thing : Entity {
  int m_iHealth;
  Weapon m_Weapon;
  Thing(std::string, double, double);
};


// OP's code starts here
Entity::Entity(string sNames, double xcord, double ycord)
    :m_sName(sNames), m_dX(xcord),m_dY(ycord){}

Thing::Thing(string sName, double xcord, double ycord):
    Entity(sName, xcord, ycord),
    m_iHealth(100),m_Weapon(Weapon("Fists", false, 10.0, 5, 1.0, xcord, ycord)){}

Weapon::Weapon(string sName, bool iMagical, double dRange, int iDamage,double
    dRadius, double dSpawnX, double dSpawnY):
    Entity(sName, dSpawnX, dSpawnY), m_bMagical(iMagical), m_dRange(dRange), m_iDamage(iDamage),
    m_dRadius(dRadius)
{
}
// OP's code ends here

int main () {
  Thing th;
}

Under g++, the precise error is:

th.cc: In function ‘int main()’:
th.cc:40: error: no matching function for call to ‘Thing::Thing()’
th.cc:27: note: candidates are: Thing::Thing(std::string, double, double)
th.cc:16: note:                 Thing::Thing(const Thing&)

Answer 2

Assuming Thing::m_Weapon is declared as Weapon and not as Weapon* , Weapon& , or equivilent, then you need to change this:

m_Weapon(Weapon("Fists", false, 10.0, 5, 1.0, xcord, ycord))

To this

m_Weapon("Fists", false, 10.0, 5, 1.0, xcord, ycord)

You are constructing a temp Weapon and then trying to construct m_Weapon from that temp. Presumably Weapon does not have a copy constructor that takes a Weapon as input.