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In C++, I'm having trouble with pointers etc. How can I fix the following problem?

error: no match for 'operator=' in '(stage->Stage::tiles + ((unsigned int)(((unsigned int)t) * 12u))) = (operator new(12u), (, ((Tile*))))'| note: candidates are: Tile& Tile::operator=(const Tile&)|*

stage.h

#include "Tile.h"

class Stage {
    public:
        Tile *tiles;
        int size;
        void init(int size);
};

stage.cpp

void Stage::init(int size) {
    this->size = size;
    this->tiles = new Tile[size];
}

application.cpp

#include "Stage.h"
#include "Tile.h"

bool setTiles( Stage * stage ) {

    for( int t = 0; t < stage->size; t++ ) {
        stage->tiles[t] = new Tile();
    }

    return true;
}

stage.init(1234);
setTiles( &stage );

Also, I don't really know when to use object.attribute and when to use object->attribute?

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8
  • Stage doesn't have a member variable called size. Please post your actual code. Commented Aug 19, 2011 at 13:07
  • Sorry, I made this version particularly so that its easier to understand. Fixed it now. Commented Aug 19, 2011 at 13:10
  • 2
    Recovering Java programmer? If you just want tiles to be an array of tiles, you don't have to do anything beyond new Tile[size] -- that already invokes the default constructors for every element in the array. The entire setTiles function as you wrote it seems to be unnecessary. Commented Aug 19, 2011 at 13:12
  • 2
    Please use std::vector! You will save yourself a lot of hassle! Commented Aug 19, 2011 at 13:13
  • @Ferdinand Beyer- and if a dependency on Boost isn't an issue, take it a step further and use boost::ptr_vector. Commented Aug 19, 2011 at 14:45

4 Answers 4

Reset to default
6 
stage->tiles[t] = new Tile();

You're calling new on something that's not a pointer. True, tiles is a pointer to an array, however, each element of that array is NOT a pointer. In order for that work, you would need an array of pointers, or a pointer to a pointer ,such as:

Tile **tiles;

What you could also do is create a separate pointer object, allocate it, and then copy the data to your array element by using

stage->tiles[i] = *somePointer;

and then deleting the pointer afterwards to free that allocated memory. This will preserve the copy because you invoked the copy constructor.

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Comments

3

You are trying to allocate a pointer with a pointer to an array. Try this one:

class Stage {
    public:
        Tile **tiles;
        void init(int size);
};
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13 Comments

Where is he "trying to allocate a pointer with a pointer to an array"?
@Oli Charlesworth This is actually pretty accurate (at least his fix is, his explanation isn't), read my answer. +1
@Oli Charlesworth: new Tile[size] creates a pointer to an array of Tile with size elements Tile *tiles is a array or a pointer to ONE Tile
@MGZero: Ok, but this answer is not complete (and neither is yours, actually). The assignment in init() needs to become this->tiles = new Tile*[size]
@Ben: C++ is very complex and as a beginner, there are many pitfalls, especially when using dynamic memory. I would strongly recommend a good book; recommendations can be found at stackoverflow.com/questions/388242/… . For a basic tutorial, consider cplusplus.com/doc/tutorial
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1 
stage->tiles[t] = new Tile();

The above is not a valid C++ code, which you are perhaps confusing with the way new is used in other language such as C#. Though new can be used to allocate dynamic memories, but assigning an object to a particular element in the dynamically created array doesn't need the new construct. In fact, the object is already created as soon as you called new Tile[size]. What you may want to do is, create an object of type Tile and assign it to a particular element in tiles.

Tile myTile;

// do something with myTile

this->tiles[0] = myTile;
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2 Comments

Well, I need to call the constructor of Tile with some arguments, i.e. I need to do Tile myTile = new Tile(...) which won't work: error: conversion from 'Tile*' to non-scalar type 'Tile' requested --- coming from Java btw :)
@Ben: I understand you had background on C# or Java :). For your case, simply use this->titles[0] = Tile(...) ;)
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new Tiles() returns a pointer to a Tiles instance.

Tile *tiles defines an array out Tiles, not pointers.

Start with Tile **tiles instead.

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