What is a properly way to iterate a char* string? Getting "corrupt" values

Question

When I was trying to iterate a number (transformed to binary with bitset library) I managed this solution

#include <iostream>
#include <bitset>

void iterateBinary(int num) {
    char *numInBinary = const_cast<char*>(std::bitset<32>(num).to_string().c_str());

    for (int i = 31; i >= 0; i--) {
        char bit = numInBinary[i];
        std::cout << i << ": " << bit << std::endl;
    }
}

But founded those weird characters in the output

I already implemented a solution to my initial idea with for (char bit : numInBinary) and without c_str() transformation, but I'm still curious about what happened (maybe memory problems?) or which can be a better way to iterate a char* string

Also remark that the "corrupt" values in the output are no the same on each ejecution and only appears at the end, why? Thanks in advance

Most of time when you need to use `const_cast`. It might be a [wrong usage](https://stackoverflow.com/q/2673508/4123703) — Louis Go, May 18 '22 at 05:38
Does this answer your question? [wierd output using const char\*, to\_string() and c\_str()](//stackoverflow.com/q/31690826/90527) — outis, May 18 '22 at 05:46

score 6 · Accepted Answer · answered May 18 '22 at 05:30

The lifetime of the string returned by to_string(), and into which the pointer returned by c_str() points, ends at the end of the full expression.

This means after the line

char *numInBinary = const_cast<char*>(std::bitset<32>(num).to_string().c_str());

the pointer numInBinary will be dangling and trying to access through it will result in undefined behavior.

You need to store the return value from to_string() so that it lives long enough, e.g.

auto numInBinary = std::bitset<32>(num).to_string();

There is also no need for a char* pointer, since std::string can just be indexed directly.

Also, if you think you need to use const_cast anywhere, rethink it. Except for very specific scenarios where you take care of const correctness by unusual means, const_cast is almost surely the wrong approach and likely to result in undefined behavior down the line.

What is a properly way to iterate a char* string? Getting "corrupt" values

1 Answers1