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Comments on constructor in C
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constructor in C
+2
−4
#include<stdio.h>
struct Book {
char title[20];
char author[20];
int pages;
};
void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
aTitle->title;
aAuthor->author;
aPages->pages;
printf("Title : %s; Author : %s; Pages : %d;",title,author,pages);
}
void main(){
struct Book title; struct Book author; struct Book pages;
struct A *c = malloc(sizeof(struct Book));
init_Book_types("Harry","JK",500);
init_Book_types("Lord","Tolkein",450);
}
In Polytechnic, teachers teach student constructor in C also. I didn't know that constructor is possible in C. Yesterday I had read in a SO question
C (as many other languages) can still be used for object oriented programming.
When I first read that Polytechnic book lot of syntax were weird to me. When I looked at the code multiple times then, I understood the system of constructor.
In cpp,
#include<iostream>
using namespace std;
class Book{
public:
string title;
string author;
int pages;
Book(string aTitle, string aAuthor, int aPages){
title = aTitle;
author = aAuthor;
pages = aPages;
cout << "Title : " << title << "; Author : " << author << "; Pages : " << pages << "\n";
}
};
int main(){
Book book1("Harry","JK",500);
Book book2("Lord","Tolkein",450);
return 0;
}
I was trying ot do the same thing in C (which code I had added at top). But, that code returns huge error.
constructor.c: In function ‘init_Book_types’:
constructor.c:14:55: error: ‘title’ undeclared (first use in this function); did you mean ‘aTitle’?
14 | printf("Title : %s; Author : %s; Pages : %d;",title,author,pages);
| ^~~~~
| aTitle
constructor.c:14:55: note: each undeclared identifier is reported only once for each function it appears in
constructor.c:14:61: error: ‘author’ undeclared (first use in this function); did you mean ‘aAuthor’?
14 | printf("Title : %s; Author : %s; Pages : %d;",title,author,pages);
| ^~~~~~
| aAuthor
constructor.c:14:68: error: ‘pages’ undeclared (first use in this function); did you mean ‘aPages’?
14 | printf("Title : %s; Author : %s; Pages : %d;",title,author,pages);
| ^~~~~
| aPages
constructor.c: In function ‘main’:
constructor.c:19:23: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
19 | struct A *c = malloc(sizeof(struct Book));
| ^~~~~~
constructor.c:2:1: note: include ‘<stdlib.h>’ or provide a declaration of ‘malloc’
1 | #include<stdio.h>
+++ |+#include <stdlib.h>
2 |
constructor.c:19:23: warning: incompatible implicit declaration of built-in function ‘malloc’ [-Wbuiltin-declaration-mismatch]
19 | struct A *c = malloc(sizeof(struct Book));
| ^~~~~~
constructor.c:19:23: note: include ‘<stdlib.h>’ or provide a declaration of ‘malloc’
constructor.c:20:25: warning: passing argument 1 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
20 | init_Book_types("Harry","JK",500);
| ^~~~~~~
| |
| char *
constructor.c:9:35: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:20:33: warning: passing argument 2 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
20 | init_Book_types("Harry","JK",500);
| ^~~~
| |
| char *
constructor.c:9:55: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~~
constructor.c:20:38: warning: passing argument 3 of ‘init_Book_types’ makes pointer from integer without a cast [-Wint-conversion]
20 | init_Book_types("Harry","JK",500);
| ^~~
| |
| int
constructor.c:9:76: note: expected ‘struct Book *’ but argument is of type ‘int’
9 | ok_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:21:25: warning: passing argument 1 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
21 | init_Book_types("Lord","Tolkein",450);
| ^~~~~~
| |
| char *
constructor.c:9:35: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:21:32: warning: passing argument 2 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
21 | init_Book_types("Lord","Tolkein",450);
| ^~~~~~~~~
| |
| char *
constructor.c:9:55: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~~
constructor.c:21:42: warning: passing argument 3 of ‘init_Book_types’ makes pointer from integer without a cast [-Wint-conversion]
21 | init_Book_types("Lord","Tolkein",450);
| ^~~
| |
| int
constructor.c:9:76: note: expected ‘struct Book *’ but argument is of type ‘int’
9 | ok_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
I had wrote code by help of the answer.
After solving few problems my code look like this
#include<stdio.h>
struct Book {
char title[20];
char author[20];
int pages;
};
void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
aTitle->title;
aAuthor->author;
aPages->pages;
printf("Title : %s; Author : %s; Pages : %d;",aTitle,aAuthor,aPages);
}
void main(){
struct Book title; struct Book author; struct Book pages;
init_Book_types("Harry","JK",500);
init_Book_types("Lord","Tolkein",450);
}
Error:
constructor.c: In function ‘main’:
constructor.c:19:25: warning: passing argument 1 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
19 | init_Book_types("Harry","JK",500);
| ^~~~~~~
| |
| char *
constructor.c:9:35: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:19:33: warning: passing argument 2 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
19 | init_Book_types("Harry","JK",500);
| ^~~~
| |
| char *
constructor.c:9:55: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~~
constructor.c:19:38: warning: passing argument 3 of ‘init_Book_types’ makes pointer from integer without a cast [-Wint-conversion]
19 | init_Book_types("Harry","JK",500);
| ^~~
| |
| int
constructor.c:9:76: note: expected ‘struct Book *’ but argument is of type ‘int’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:20:25: warning: passing argument 1 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
20 | init_Book_types("Lord","Tolkein",450);
| ^~~~~~
| |
| char *
constructor.c:9:35: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
constructor.c:20:32: warning: passing argument 2 of ‘init_Book_types’ from incompatible pointer type [-Wincompatible-pointer-types]
20 | init_Book_types("Lord","Tolkein",450);
| ^~~~~~~~~
| |
| char *
constructor.c:9:55: note: expected ‘struct Book *’ but argument is of type ‘char *’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~~
constructor.c:20:42: warning: passing argument 3 of ‘init_Book_types’ makes pointer from integer without a cast [-Wint-conversion]
20 | init_Book_types("Lord","Tolkein",450);
| ^~~
| |
| int
constructor.c:9:76: note: expected ‘struct Book *’ but argument is of type ‘int’
9 | void init_Book_types(struct Book* aTitle,struct Book* aAuthor,struct Book* aPages){
| ~~~~~~~~~~~~~^~~~~~
Post
+8
−0
The warnings just say that you can't pass a string literal with type char[] to a function taking a struct Book* parameter. The function should be declared as:
void init_Book_types (const char* title, const char* author, int pages);
And in the constructor, aTitle needs to be aTitle->title when you call printf.
All string assignments have to be carried out with strcpy since C doesn't have a string class.
General advise when dealing with compiler messages is to deal with them one at a time. Fix the first on in the list, re-compile, then fix the next.
However, this is not how you do proper OO in C. You can do it, but it is somewhat cumbersome. Since you are a beginner, I'd simply advise to stay clear of OO for now. If you are curious still, there's some example here.
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