How to use scanf and printf for string in C language

Question

I don't want to declare character array. I want to declare string. And take input and show output of that string using scanf and printf. I have tried to do it this way that's shown below but it doesn't work properly. How to do it in proper way?

#include<bits/stdc++.h>
using namespace std;

int main()
{
    string str;

    scanf("%s", str.c_str());
    printf("%s", str.c_str());

    return 0;
}

When I take input using this code, output shows like this

C or C++? C does not have strings - only nul terminated char arrays. So you must use a character array. — Weather Vane
– Weather Vane, Commented Feb 12, 2018 at 12:44
str.c_str() is read-only. And not allocated/empty... UB. use std::cin instead. std::string supports it natively — Jean-François Fabre
– Jean-François Fabre ♦, Commented Feb 12, 2018 at 12:47
You are not supposed to mix C and C++ like that. It's basically impossible to make the scanf line correct because it reads an arbitrary amount of data into a buffer, so you need an arbitrarily big buffer which means you must make the buffer bigger while reading. scanf doesn't do that. std::cin does so you can do std::cin >> str;. You should try to use the tools that do exactly what you want instead of hacking on tools that are made for a different purpose. — nwp
– nwp, Commented Feb 12, 2018 at 12:50

cydef · Accepted Answer · 2018-02-12 13:22:24Z

1

You are trying to write in a zone which is read only e.g. str.c_str(). What you want to do is either :

 std::cin >> str;

or if for a specific reason you have to use scanf. Since C++17 you can do :

  str.data()

Which returns a modifiable memory zone.

Edit : as @nwp said, you have to call str.resize(size) to because the size of the string is 0 before that.

edited Feb 12, 2018 at 13:22

answered Feb 12, 2018 at 12:53

cydef

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2 Comments

nwp Over a year ago

Modifiable yes, but with size 0, so not very useful. You would need to call str.resize(some_magic_number); first.

cydef Over a year ago

@nwp True, I don't see the point of using scanf here so I omitted that detail. Thanks I'll edit to match your comment

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