NON-SCALAR TYPE CONVERSION!?

Jorgmen

Sorry you all for bothering on Sunday: I'm new using C++ and I need to simulate a Social network, but I'm having troubles doing a "member": this is the code is very simple: I hava a header called "structs.h"

struct Miembro{
int id;
int activo;
int pass;

Miembro(int a, int b, int c)
{
id = a;
activo = b;
pass = c;
}
};

//******************************
//Main is like:

#include #include #include #include "structs.h"

using namespace std;

int main(void)
{

cout<< "hola";

string nombre;
nombre = "Jorge";
cout <<" "<< nombre << endl;

Miembro memb = new Miembro(1, 1, 123);

// system("pause");
// return 0;
}

F:\main.cpp In function 'int main()': 17 37 F:\main.cpp [Error] conversion from 'Miembro*' to non-scalar type 'Miembro' requested F:\Makefile.win [Error] [main.o] Error 1 (if this is the only error: please check your library includes)

jorgmen

enhzflep

Because you're creating a new Miembro in this way, all you get back is a pointer to it. You then try to assign this pointer to an object of type Miembro. Just change it to this:

Miembro *memb = new Miembro(1,1,123);

Make it work. Then do it better - Andrei Straut

pasztorpisti

OR, you can just say

Miembro memb(1, 1, 123);

and this will allocate the memb object on the stack. I think this is the recommended way to allocate small objects whenver possible. Objects allocated on the stack are deleted automatically for you when they run out of scope (usually at the end of your function or if-block or whatever). If you allocate an object from the heap then you get a pointer to the heap and you must delete that object via its pointer when you done with it using the delete keyword. By forgetting to delete heap objects your program might continuously 'eat' the memory until you exit the program (this memory waste is called 'memory leak').

Jorgmen

Thaks for your replays: now i have a new q?

struct Miembro{
int id;
int activo;
int pass;

Miembro(int a, int b, int c)
{
id = a;
activo = b;
pass = c;
}
};

struct NodoMiembro{
NodoMiembro * nextmiem;
NodoMiembro * prevmiem;
Miembro persona;

NodoMiembro(Miembro nuevo){
nextmiem = prevmiem = NULL;
persona = nuevo;
}
};

struct ListaMiembros{
NodoMiembro * lastmember;
NodoMiembro * firstmember;

ListaMiembros(){
lastmember = firstmember = NULL;

}

I can't compile this, I'm having headache because of c++: it says: 4 0 F:\main.cpp In file included from main.cpp F:\structs.h In constructor 'NodoMiembro::NodoMiembro(Miembro)': 31 27 F:\structs.h [Error] no matching function for call to 'Miembro::Miembro()' 31 27 F:\structs.h [Error] candidates are: 17 1 F:\structs.h Miembro::Miembro(int, int, int) 17 1 F:\structs.h candidate expects 3 arguments, 0 provided 12 8 F:\structs.h Miembro::Miembro(const Miembro&) 12 8 F:\structs.h candidate expects 1 argument, 0 provided F:\structs.h At global scope: WHY NodoMiembro can't have a "Miembro xxx" as parameter in the constructor???

enhzflep

Ooo-ouch! Those warning messages are pretty nasty. They offered me no help at all in solving the problem. At first I thought it may be a result of being structs, rather than classes - no, of course not. Default access is public instead of private, but it did then get me thinking. I was writing out the constructors when the idea of default constructors hit me in the face.. Notice in your NodoMiembro definition, you have a variable of type Miembro? Well, have a close look at the way it's defined - It's just Miembro persona; Just back-up a bit and think what would happen if you tried to instantiate the struct like this in your main() - it would fail miserably. Here:

#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;

struct Miembro
{
int id;
int activo;
int pass;

Miembro(int a, int b, int c)
{
    id = a;
    activo = b;
    pass = c;
}

};
int main(void)
{
Miembro a;
}

Basically, the fix is to add default constructors to both the the Miembro and the NodoMiembro structs.

Miembro()
{
id=activo=pass=0;
}

and

NodoMiembro()
{
nextmiem = prevmiem = NULL;
}

Make it work. Then do it better - Andrei Straut

Chuck OToole

The declaration

Miembro persona;

is the same as saying

Miembro persona();

(constructor with 0 arguments) but the only constructor you defined requires 3 arguments

Miembro(int a, int b, int c)

so the compiler cannot resolve the reference.

pasztorpisti

Chuck O'Toole wrote:

Miembro persona;

is the same as saying

Miembro persona();

Just want to mention an exotic exception: If Miembro was a POD type then the two statements had different effects. The first wouldn't initialize the variable while the second would zero initialize. PODs are primitive types - ints, bools, etc... and structs/classes without any explicitly declared constructors/destructor/virtual methods (briefly: C-struct compatible stuff).

pasztorpisti

Jorgmen wrote:

Sorry you all for bothering on Sunday

Those who don't want to be bothered simply don't login on sunday... :-)

_Superman_

Not a reply to your question, but just for completeness. Always prefer initialization to assignment for member variables. What you've done in the constructor is assignment. Do the following for initialization instead -

Miembro(int a, int b, int c) : id(a), activo(b), pass(c)
{
}

«_Superman_» _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++)

Polymorphism in C