C code does not work when coded like C++

Question

Hello Developers! I am learning algorithms from Algorithms Design Manual Book by Skiena. There I have the following code:

    #include <stdio.h>
#include <stdlib.h>

typedef int item_type;

typedef struct{
    item_type item;
    struct list* next;
    }list;

void insert_list(list **l, item_type x){
    list *p;
    p = malloc(sizeof(list));
    p->item = x;
    p->next = *l;
    *l = p;
    }

int main(){
    return 0;
    }

It gives me Warning when compiled:

gcc -Wall -o "test" "test.c" (in directory: /home/akacoder/Desktop/Algorithm_Design_Manual/chapter2) test.c: In function ‘insert_list’: test.c:15: warning: assignment from incompatible pointer type Compilation finished successfully.

But when I rewrite this code as C++:

 #include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

typedef int item_type;

typedef struct{
    item_type item;
    struct list* next;
    }list;

void insert_list(list **l, item_type x){
    list *p;
    p = malloc(sizeof(list));
    p->item = x;
    p->next = *l;
    *l = p;
    }

int main(){
    return 0;
    }

It gives the following:

g++ -Wall -o "chapter2" "chapter2.cpp" (in directory: /home/akacoder/Desktop/Algorithm_Design_Manual/chapter2) chapter2.cpp:15: error: conflicting declaration ‘typedef struct list list’ chapter2.cpp:14: error: ‘struct list’ has a previous declaration as ‘struct list’ chapter2.cpp: In function ‘void insert_list(list**, item_type)’: chapter2.cpp: In function ‘void insert_list(list**, item_type)’: chapter2.cpp:19: error: invalid conversion from ‘void*’ to ‘list*’

Can anyone explain why it is so? And How can I rewrite it in C++?

Answer 1

This is because c++ is stricter than c with respect to type conversions.

There are host of other errors in your code. Please note that just putting a c source code, renaming the file as .cpp & compiling using g++ does not make a c source code as c++.

If you are writing a program in c++ please use new & not malloc, doing so you do not need to explicitly type cast as in case of malloc.

Answer 2

Your problem in both cases is in the struct definition: struct list *next doesn't refer to the struct you are in the process of declaring. Try this instead:

typedef struct list {
    item_type item;
    struct list* next;
} list;

In addition, in C++ you must cast the void * returned by malloc to the appropriate pointer type (list *), C++ is stricter about these things. Also, BTW, in C++ you can leave off the typedef completely if you want.

The reason for the differing error messages is a difference in the languages.

In C, the compiler knows that struct list * is a pointer to a struct, so it doesn't need to complain that it doesn't actually know what a "struct list" is yet. Later, though, when you try to assign this "struct list *" from a pointer of type "list *" (the type of which is "pointer to an anonymous struct"), it complains about the mismatch.

In C++, a "struct" declaration is more or less equivalent to a "class" declaration (the major difference is in the default visibility of members). Among other things, this means that structs in C++ are more or less automatically typedefed. So when the compiler sees "struct list *next", it takes it as a forward declaration of a class named "list"; then when it finishes the statement and processes the typedef, throws an error because you're trying to typedef something to an identifier that is already (forward-)declared as something else. Then it issues further errors because it doesn't actually know what "list" might be, due to the earlier error.