Replace a part of a string [closed] - c++

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In my code I have edited the string at an adneren position, this I have not had in my example.
Since here the actual error cause/problem lay I have adapted the example, see example below.
The Answer should be:
std::string.resize increases the memory size of the string to the maximum expected size.
replace inserts the entire size into str0.
A solution is to execute resize only if str1.lengh > max_len.
Original question:
I would like to insert a string (str1) into another (str0), at any position (n).
If str1 is shorter than the rest of string str0 (n -> str0.end()), the rest of str0 should remain.
For this I wanted to use the std::string::replace() function.
#include <string>
#include <stdint.h>
#include <iostream>
int main()
{
std::string str0 = "ABCDEFGHIJ";
std::string str1 = "000";
uint16_t n = 3;
str0.replace(n, str1.length(), str1);
std::cout << "TEST:" << str0 << std::endl;
return 0;
}
In my example, I expect to output "TEST: ABC000GHIJ" via cout.
However, "TEST: ABC000" is output.
Where is the problem that the string will be cutted?
Correct code example with the error cause:
#include <string>
#include <stdint.h>
#include <iostream>
#define max_len 20
int main()
{
std::string str0 = "ABDCDEF";
std::string str1 = "";
uint16_t n = 3;
std::getline(std::cin, str1);
if(!str1.empty() && str1[str1.size()-1] == '\n') {
str1.erase(str1.size()-1);
}
str1.resize(max_len);
str0.replace(n, str1.length(), str1);
std::cout << "TEST:" << str0 << std::endl;
return 0;
}