String stores data from unrelated memory address

Question

I'm trying to generate a string of 50 dots. I have the following code:

#include <string> using namespace std; int main(){ string s(".",50); cout << s; }

and the output is:

.vector::_M_realloc_insert$C����pX�%

The string s doesn't store . 50 times, but some other part of memory. A leak happened, and I have no idea how.

What went wrong? How do I go about generating a string of length n consisting only of dots? In general, I want to do in c++ what in python would be done by "c"*n.

Thank you.

You should read the documentation more carefully. The constructor you want is std::string s(50, '.');. And next time, please remove anything irrelevant (i.e. 95% of it) from the code you post, see minimal reproducible example. — catnip
– catnip, Commented Dec 22, 2022 at 22:32
ok ty. i didnt know what was the minimal reproducible data since the print out referred to vec, etc. removed everything else. — subjectsphinx
– subjectsphinx, Commented Dec 22, 2022 at 22:44

Some programmer dude · Accepted Answer · 2022-12-22 22:33:40Z

Your example could be a whole lot shorter, and still have the same problem.

The problem is that with the std::string constructor you use, the length is the length of the string ".", not how long the std::string should be. Since "." is not 50 characters long, you will have undefined behavior.

From the linked reference for constructor number 4 (the one you use):

The behavior is undefined if [s, s + count) is not a valid range

There s is the string "." and count is the value 50.

The constructor I guess you want to use is the one taking a single character instead of a string (number 2 in the linked reference):

std::string s(50, '.'); // Fill string with 50 dots

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