Question

Here is my code:

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


using namespace std;

struct Computer
{
    char * model;
    char * assembler;
    int processorInt;
};

int main()
{
    Computer comp;
    char* model;
    char* assembler;
    int processorInt;

    cin>>model;
    cin>>assembler;
    cin>>processor int;

    comp.model = model;
    comp.assembler = assembler;
    comp.processorInt = processorInt;

    return 0;
}

//if i do so, it works, BUT if i do in another way, it gives Segmentation Fault

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


using namespace std;

struct Computer
{
    char * model;
    char * assembler;
    int processorInt;
};

void setValues()
{
    Computer comp;
    char* model;
    char* assembler;
    int processorInt;

    cin>>model;
    cin>>assembler;
    cin>>processor int;

    comp.model = model;
    comp.assembler = assembler;
    comp.processorInt = processorInt;
}

int main()
{
    setValues();

    return 0;
}

So what’s the reason?

My goal is to create an array of structures in which i can save some info about each “computer”, then make a possibility to edit any structure and then sort the whole array by procesorInt. But I can’t even create a normal editable structure.

Answer 1

Neither version works, you’re trying to read data into an uninitialized pointer. Just declaring e.g. char* model; leaves you with a pointer that isn’t initialized: it can point to any spot in memory. When you try to store a string at that location using cin >> you’ll be writing memory that doesn’t belong to you. This might segfault, or it might appear to work.

Just declaring char* doesn’t give you space for the string: if you’re using standard C strings then you’ll need to give them some data: make your char pointers arrays of a fixed size, or use malloc to allocate a string buffer:

char model[MAX_STRING_LENGTH];

Or use malloc, but be aware that you’ll need to free this memory later:

char *model = malloc(MAX_STRING_LENGTH);

Now, if you’re going to use standard C strings and cin together, you shouldn’t use >> at all: there’s no way to limit the input size. Instead use cin.getline with MAX_STRING_LENGTH as the limit (see the example in the docs for details).

But definitely prefer to use std::string instead: if you do so then you won’t need to deal with providing space for the string yourself, and your code won’t need to change much at all.