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, May 30 '22 at 14:04
And you simply cannot directly write to a `std::string` via C IO functions — UnholySheep, May 30 '22 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, May 30 '22 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, May 30 '22 at 14:09
Does this answer your question? [Read into std::string using scanf](https://stackoverflow.com/questions/20165954/read-into-stdstring-using-scanf) — Blackgaurd, May 30 '22 at 14:13

score 2 · Answer 1 · answered May 30 '22 at 14:18

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.

Note, also, that if `scanf` reads, for example, only one character, `s.size()` is still 4096. +1. — Pete Becker, May 30 '22 at 14:33
Okay thank you for the answer and your suggestion churill and pete becker. I decide to use cin because that is still not work — Randy Gunawan, May 30 '22 at 14:49

input std string in c++ with scanf

1 Answers1