stl list of structs

stl list of structs

Frank Drebin

Member #2,987

December 2002

until now i only used classes with stl::list.
when using a stl::list of mystruct the compiler gives me an error if i do

mystructs.push_back(); or mystructs.push_back(mystruct);

it seems i have to write

mystructs.push_back(mysrtuct());

is this ok?
why do i need the brackets here - a struct doens't have a construcor, right?

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juvinious

Member #5,145

October 2004

Hmm, what?
you mean something like:

typedef struct myStruct
{
        int x;
        int y;
}MYSTRUCT;

list<MYSTRUCT>sList;
MYSTRUCT obj;
sList.push_back(obj);

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Frank Drebin

Member #2,987

December 2002

yes... but it won't compile this way

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juvinious

Member #5,145

October 2004

compiles for me

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Paintown

Archon

Member #4,195

January 2004

Quote:

a struct doens't have a construcor, right?

Isn't a struct in C++ just a class with default protection levels to public (IIRC from another thread in the past)?

Hano Wair

Member #5,243

November 2004

Quote:

Isn't a struct in C++ just a class with default protection levels to public (IIRC from another thread in the past)?

It is.

Frank Drebin

Member #2,987

December 2002

!
so how's a constructor defined in a struct (example)?
is there a destructor, too?

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Archon

Member #4,195

January 2004

Quote:

so how's a constructor defined in a struct (example)?
is there a destructor, too?

Try defining it like you would define a class...

CGamesPlay

Member #2,559

July 2002

There is absolutely no difference between this line:struct A {And this line:class A { public:

[edit]
s/C/A/

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Frank Drebin

Member #2,987

December 2002

1#include <stdio.h>
2 
3struct test
4{
5 test() { printf("Constructor\n"); }
6 ~test() { printf("Destructor\n"); }
7};
8 
9 
10int main()
11{
12   test mytest; // works
13   // test mytest(); //doesn't work
14 
15   return 0;
16}

why aren't the *structors called when i put brackets after mytest?

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CGamesPlay

Member #2,559

July 2002

Interesting. I'm sure it has something to do with this problem. The "fix" is this program:

1#include <stdio.h>
2 
3struct test
4{
5    test()
6    { printf("Constructor\n"); }
7    ~test()
8    { printf("Destructor\n"); }
9};
10 
11 
12int main()
13{   
14    printf("no paren\n");
15    test mytest;
16    printf("paren\n");
17    test mytest2 = test();
18    printf("new\n");
19    test* mytest3 = new test();
20    delete mytest3;
21 
22    return 0;
23}

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ImLeftFooted

Member #3,935

October 2003

Quote:

There is absolutely no difference between this line:

struct A {

And this line:

class C { public:

But A and C are different

How is my posting?

CGamesPlay

Member #2,559

July 2002

Heh, interesting typo.

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Indeterminatus

Member #737

November 2000

Quote:

// test mytest(); //doesn't work

It doesn't work because it's a forward declaration of a function with the name mytest.

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Matthew Dalrymple

Member #7,922

October 2006

Shouldn't this work though?

test *mytest = new test();

=-----===-----===-----=
I like signatures that only the signer would understand. Inside jokes are always the best, because they exclude everyone else.

Archon

Member #4,195

January 2004

Quote:

Shouldn't this work though?

test *mytest = new test();

Yeah but that's using the heap memory.

X-G

Member #856

December 2000

Quote:

There is absolutely no difference between this line:

Wrong!

Structs have public inheritance by default. Classes do not.

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CGamesPlay

Member #2,559

July 2002

Quote:

Structs have public inheritance by default. Classes do not.

Could you post a code segment demonstrating the difference?

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X-G

Member #856

December 2000

1#include <stdio.h>
2 
3struct Primate {
4  int legs;
5};
6struct Monkey : Primate {
7};
8 
9class Vegetable {
10public:
11  int leaves;
12};
13class Tomato : Vegetable {
14};
15 
16int main(int argc, char **argv) {
17  Monkey steve;
18  Tomato pudgey;
19  
20  printf("Steve has %d legs and Pudgey has %d leaves\n", steve.legs, pudgey.leaves);
21  
22  return 0;
23}

gcc said:

classes.cpp: In function `int main(int, char**)':
classes.cpp:11: error: `int Vegetable::leaves' is inaccessible
classes.cpp:20: error: within this context

Indeterminatus

Member #737

November 2000

Quote:

Structs have public inheritance by default. Classes do not. [emphasis added, ed.]

struct Base {
};

struct A : Base { // public inheritance by default
};

class WhatEverBase {
};

class A : public WhatEverBase { // not public by default
};

Granted, it doesn't make a difference in the sample you posted (as there's no inheritance whatsoever), but it still has to be considered a difference between a class and a struct (their default visibility aside).

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CGamesPlay

Member #2,559

July 2002

Okay. That's what I thought, but I didn't know that struct A : Base was valid C++

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