input std string in c++ with scanf

Question

Why the result of my code is not true. Is it because scanf function?

#include <cstdio>
#include <string>

using namespace std;

int main(){
    string s, t;
    scanf("%s", s.c_str());
    scanf("%s", t.c_str());
    int a = s.find(t);
    while(a > -1){
        s = s.erase(a, t.length());
        a = s.find(t);
    }
    printf("%s", s.c_str());
}

That should give you compile warnings to start with. c_str returns a const char* while %s expects a non-const pointer — UnholySheep
– UnholySheep, Commented May 30, 2022 at 14:04
And you simply cannot directly write to a std::string via C IO functions — UnholySheep
– UnholySheep, Commented May 30, 2022 at 14:05
There is no reason to assume that strings s and t have buffers large enough to take the scanned string. — Mark Roberts
– Mark Roberts, Commented May 30, 2022 at 14:05
why do you want to do this instead of the perfectly legal and working C++ std::cin >> s >> t; ? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 30, 2022 at 14:09
Does this answer your question? Read into std::string using scanf — Blackgaurd
– Blackgaurd, Commented May 30, 2022 at 14:13

Lukas-T · Accepted Answer · 2022-05-30 14:18:00Z

2

scanf will simply write to any buffer you give it. It doesn't know that the buffer belongs to a std::string, nor does it know how large the buffer is. Also you are writing to a const char* - that points to a readonly buffer of unknown size. Your code has undefined behaviour.

Yes, it is possible to directly write to the internal buffer of a std::string using std::string::data(). But when you do this, you need to make sure the buffer is large enough using resize() or constructor #2:

int main(){
    string s(4096, '\0');
    scanf("%s", s.data());
    ...
}

You end up with the same problem as if you were using a plain char[].

The C++-way std::cin >> s; or std::getline(std::cin, s) would be much easier and safer.

answered May 30, 2022 at 14:18

Lukas-T

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2 Comments

Pete Becker Over a year ago

Note, also, that if scanf reads, for example, only one character, s.size() is still 4096. +1.

Randy Gunawan Over a year ago

Okay thank you for the answer and your suggestion churill and pete becker. I decide to use cin because that is still not work

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