Constructor w/Multiple Arguments in Constructor list not working C++

Question

I'm trying to compile class A, which has a member of class B, where class B has no default constructor and its only constructor requires multiple arguments. Simple, right? Apparently not...

Class A:

class SessionMediator
{
 public:
  SessionMediator()
      : map_(16,100,100)
  {}

  Tilemap map_, background_, foreground_;
};

Class B:

struct Tile2D;

class Tilemap
{
 public:
  Tilemap(const unsigned int tile_size, const unsigned int width, 
          const unsigned int height)
      : tiles_(NULL), tile_size_(tile_size)
  {
    Resize(width, height);
  }

  inline void Resize(const unsigned int width, const unsigned int height)
  { /* Allocate tiles & assign to width_, height_... */ }

  unsigned int tile_size_, width_, height_;
  Tile2D* tiles_;
};

I am instantiating SessionMediator like so:

int main(int argc, char** argv)
{
  SessionMediator session;
  return 0;
}

This is the error I am getting. I'm compiling in XCode on Mac OS 10.5.8 and the compiler is g++:

session_mediator.h: In constructor 'SessionMediator::SessionMediator()':
session_mediator.h:19: error: no matching function for call to 'Tilemap::Tilemap()'
tilemap.h:31: note: candidates are: Tilemap::Tilemap(unsigned int, unsigned int, unsigned int)
tilemap.h:26: note:                 Tilemap::Tilemap(const Tilemap&)
session_mediator.h:19: error: no matching function for call to 'Tilemap::Tilemap()'
tilemap.h:31: note: candidates are: Tilemap::Tilemap(unsigned int, unsigned int, unsigned int)
tilemap.h:26: note:                 Tilemap::Tilemap(const Tilemap&)

(Duplicate of above here)
Build failed (2 errors)

I wrote a short compilable example doing basically the same thing, to try to figure out what exactly I was doing wrong, which compiles just fine with no errors in g++:

class A
{
 public:
  A(int x, int y, int z)
      : x_(x), y_(y), z_(z)
  {}  

  int x_, y_, z_; 
};

class B
{
 public:
  B() 
    : m_a(1,2,3)
  {}  

  A m_a;
};

int main(int argc, char **argv)
{
  B test;
  return 0;
}

Why does it fail in the first example? The 3 arg constructor for Tilemap (in Ex#1) is being called in the same way that the 3 arg constructor for A is being called (in Ex#2).

The code seems pretty much identical to me in the two examples.

Is there any other constructors that you have? Is line 19 that particular constructor? — vmpstr, Oct 23 '12 at 19:36
There is another SO post similar to this: Declaring a member variable that takes a constructor parameter http://stackoverflow.com/a/7395203/391693 — farfareast, Oct 23 '12 at 19:36
Those are the only constructors. I'll look at the other post, I tried hard to find an answer to this question before I posted but most of what I got were basic stuff about how the constructor initialization list works. — TPXL, Oct 23 '12 at 19:40
also u r not showing how u r calling `SessionMediator` or `TileMap` like u have shown in `main` method of the example — Anirudha, Oct 23 '12 at 19:47
Adding a section showing how SessionMediator is being instantiated. It is using the default constructor. — TPXL, Oct 23 '12 at 19:53
@Fake.It.Til.U.Make.It: Days of favoring inheritance over containment are long gone :-). — farfareast, Oct 23 '12 at 20:01
Try to put `map_(16u,100u,100u)` in the SessionMediator constructor call to make the constants unsigned. This is the only thing that comes to mind now :-). — farfareast, Oct 23 '12 at 20:50
There's too much guessing going on here. You have left out some critical piece of information; edit your problem code so that it can be copy/pasted and compiled to produce the error you're running into. As currently posted (after minor fixes, such as forward declaring `Tile2D` and fixing the comment that hides a closing brace), the code code does not produce the error you're asking about with GCC/G++ or MSVC. — Michael Burr, Oct 23 '12 at 22:26

TPXL · Accepted Answer · 2012-10-24T01:50:17.300

As I was trying to simplify my example a bit, I accidentally left out two important things: other data members in the SessionMediator class.

The problem was that I had two additional members of the Tilemap class ("background_" and "foreground_"), which weren't being initialized in the constructor initialization list like the first member "map_".

The constructor should be changed to this:

SessionMediator()
    : map_(16,100,100), background_(1,1,1), foreground_(1,1,1)
{}

I apologize for any time wasted over this question; it turned out to be something much simpler. Hopefully someone else will see this question and realize the mistake they are making.

score 0 · Answer 2 · answered Oct 23 '12 at 19:42

0

The only thing I can think of is if you are using the copy constructor:

SessionMediator a = b; or SessionMediator a (b);

You might get in the situation where the default copy constructor of SessionMediator would try to use the default constructor of Tilemap which will cause the error you have.

answered Oct 23 '12 at 19:42

tozka

3,211
19
23

I'm creating the session mediator object as "SessionMediator session;", if that helps any. But I will look into that... – TPXL Oct 23 '12 at 19:45
Also check for forgotten colon, comma in the constructor or something like that, some spelling mistake. Also if they are defined in the same file, if `Tilemap` is declared after `SessionMediator` the compiler would not know about `TileMap` when it reach SessionMediator constructor and that might cause such an error. – tozka Oct 23 '12 at 20:00

farfareast · Answer 3 · 2012-10-23T20:56:20.260

0

Try to put map_(16u,100u,100u) in the SessionMediator constructor call to make the constants unsigned. This is the only thing that comes to mind now :-).

This compiles fine for me:

class Tilemap
{
public:
    Tilemap(const unsigned int tile_size, const unsigned int width, 
        const unsigned int height)
    {
    }
};

class SessionMediator
{
public:
    SessionMediator(): map_(16u,100u,100u){}

    Tilemap map_;
};

edited Oct 23 '12 at 20:56

answered Oct 23 '12 at 20:10

farfareast

2,179
1
23
22

This didn't work, but thanks for trying. I've been going over the code, I can't find any obvious syntax errors in either file. I'll let you guys know what the problem was if I find it though. – TPXL Oct 23 '12 at 21:02

score -1 · Answer 4 · answered Oct 23 '12 at 19:30

-1

Well, when you do:

 Tilemap map_;

You're calling the default ctor - but you don't have one defined, that's the error message.

On the extra:

 Tilemap::Tilemap(const Tilemap&)

C++ generates a ctor that takes a reference for you. So the valid matches are (1) the one you defined which takes 3 args and (2) the auto-generated one that takes the const ref.

answered Oct 23 '12 at 19:30

Mark Stevens

2,366
14
9

1

When he writes `Tilemap map_;`, he is declaring a member. No constructor is called until the containing class constructor is called; in which case, the constructor of the member is called with the arguments given in the initialization list. – James Kanze Oct 23 '12 at 19:39
@TragicPixel: Ahhh subtle, when you intantiate B, it calls "B() : m_a(1,2,3)" - calling the 3 arg ctor for A; if you remove the ":m_a(1,2,3)" it should fail nicely ;-) – Mark Stevens Oct 23 '12 at 19:39
@MarkStevens But I'm instantiating map_ in the same exact way as I instantiate m_a: after instantiating SessionMediator, it calls "SessionMediator() : map_(1,2,3)" calling the 3 arg ctor for Tilemap. Which is exactly what is happening in the second example. – TPXL Oct 23 '12 at 19:48
@TragicPixel: Sorry, maybe it's a compiler difference, but both examples compile correctly for me with VS2010, and both fail to compile if I remove the call to the 3 argument ctor (with the "no appropriate default constructor" error). Sorry, I think I'm out of ideas. – Mark Stevens Oct 23 '12 at 19:59
Oh well. Thanks for the effort anyway! – TPXL Oct 23 '12 at 21:00

Constructor w/Multiple Arguments in Constructor list not working C++

4 Answers4