The difference between char*s=new char[] and char s[20]?

Question

#include<iostream>
#include<process.h>
using namespace std;
template<class T>
class List{
    class Node{
    public:
        T num;
        Node*next;
    }*head,*tail;
public:
    List(){
        head = tail = NULL;
    }
    void insert(T *n){
        Node*tmp=new Node;
        tmp->next = head;
        tmp->num = *n;
        head = tmp;
        if (tail == NULL){
            tail = tmp;
        }
    }
    void append(T*n){
        Node*tmp=new Node;
        tmp->next=NULL;
        tmp->num = *n;
        if (tail == NULL){
            head = tail = tmp;
        }
        else {
            tail->next = tmp;
            tail = tmp;
        }
    }
    T Get(){
        if (head == NULL){
            exit(0);
        }
        else{
            T t = head->num;
            Node*p = head;
            if (head->next == NULL){
                head = tail = NULL;
            }
            else{
                head = head->next;
            }
            delete (p);
            return t;
        }
    }
};
class person{
public:
    char*name=new char[];//problem lies here!
    //char name[20];
    int age;
    float hight;
    person(){}
};
int main(){
    person a;
    List<int>link1;
    List<person>link2;
    for (int i = 0; i < 5; i++){
        cin >> a.name >> a.age >> a.hight;
        link2.insert(&a);
        link1.append(&i);
    }
    for (int i = 0; i < 5; i++){
        a = link2.Get();
        link2.append(&a);
        cout << a.name << " " << link1.Get() << endl;
    }
}

As the explanation goes in the code, when using char*name=new char[] to replace char name[20], the program went wrong. It wouldn't output all the names as expected, but only print the last input name 5 times. So what's the difference between these two expressions? Thanks a lot.

Don't spam language tags. C and C++ are **different** languages. — too honest for this site, Jun 03 '16 at 03:03
A question about `new` like this means you are programming in C++, not C; `new` is not an operator in C. Please don't dual tag questions with C and C++ unless the question is really about the interactions between the two languages. — Jonathan Leffler, Jun 03 '16 at 03:04
I don't think "static allocation" is a good term because it can live inside a dynamically allocated object. — , Jun 03 '16 at 03:06
People put in their CV like "Proficiency: C/C++" and most of them dont understand the difference. — awatan, Jun 03 '16 at 03:06
`char s[20];` is not static allocation, it's automatic memory allocation. Let's stop using words we don't understand. — uh oh somebody needs a pupper, Jun 03 '16 at 03:07
@sleeptightpupper I think it is more complicated than that. `char s[20];` can be static (e.g. global, static local), automatic (e.g. local), or dynamic (e.g. a class member, and then `new` the class). — , Jun 03 '16 at 03:10
Let's not quibble (troll) over words often used interchangeably [**Static memory allocation**](https://en.wikipedia.org/wiki/Static_memory_allocation) — David C. Rankin, Jun 03 '16 at 03:10
@NickyC It's not more complicated than that. All of those are mutually exclusive storage durations. — uh oh somebody needs a pupper, Jun 03 '16 at 03:12
@sleeptightpupper I am just saying this syntax may be used to do static allocation. — , Jun 03 '16 at 03:15
@DavidC.Rankin The article you link exactly opposes your opinion. — , Jun 03 '16 at 03:17

score 0 · Answer 1 · answered Jun 03 '16 at 03:11

0

new char[] doesn't compile under GCC. :) new char[20], however, does.

char[20] tells the compiler that you want 20 characters in an array. This is typically allocated on your call stack. Very large allocations can cause stack overflows so this is discouraged in some circles.

If you use new you must call delete[] name or you will have a memory leak. If you call delete name you will have an error in your code because the array will not be deleted, only that one char *.

answered Jun 03 '16 at 03:11

Sam

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17

Did you mean `g++` ? `new char[20]` will not compile under gcc, but it will under `g++` – sjsam Jun 03 '16 at 03:19
@sjsam No. GCC is the name of the actual compiler, `g++` is the C++ front-end. – uh oh somebody needs a pupper Jun 03 '16 at 03:21
@sleeptightpupper : I meant the binary, not the collection ;-) I see that \@sam has capitalized `GCC` and I think he actually meant the compiler collection itself. :-) – sjsam Jun 03 '16 at 03:26
Correct. Thank you. :) I used /usr/bin/g++ and for kicks I ran clang++, just to be sure. :) – Sam Jun 03 '16 at 14:32

The difference between char*s=new char[] and char s[20]?

1 Answers1