如何在不使用 append 函数的情况下附加到 C++ 字符串？

Question

I need to append the value of string2 to string1 without using append method in string library. So far, I've only managed to do this:

#include<iostream>
#include<string>
using namespace std;
int main(){
    string s1="Messi ";
    string s2="best";
    int i=s1.size();
    int j=s2.size();
    int count;
    for(count=0;count<=j;count++){
        s1[i++]=s2[count];

    }   
    cout<<s1;
}

But this, only gives me the value of s1 which is "Messi ". However, I checked by doing this: s1[8]. Since, s1 has index 5, s1[8] shouldn't work, right? But, it does and I get "s" which is in s2. So, it means that the value of s2 is indeed going in s1 but somehow, typing s1 only shows "Messi".

The reason I ask this because we were given this set of instructions to implement the program(yes it appears to be used on c strings but we were told to implement on string object):

Step 1: Initialize i= strlen(s1)
Step 2: Initialize j=strlen(s2)
Step 3: Initialize count=0
Step 4: Repeat steps 5 to 7 while count<=j
Step 5: s1[i]=s2[count]
Step 6: i=i+1
Step 7: count=count+1

Answer 1

I think this is what you want

#include<iostream>
#include<string>
using namespace std;
int main(){
    string s1="string1 ";
    string s2="string2";
    int i=s1.size();
    int j=s2.size();
    s1.resize(i+j);

    for(int count=0;count<=j;count++){

        s1[count+i]=s2[count];

    }   
    cout<<s1;
}

After the modifications you've made, I think the code you are looking for is as follows

#include<iostream>
#include<cstring>
using namespace std;
int main(){
    char s1[10]="str1 ";
    char s2[]="str2";
    int i=strlen(s1);
    int j=strlen(s2);
    int count = 0;
    for(;count<=j;){
        s1[i]=s2[count];
        i=i+1;
        count=count+1;

    }   
    cout<<s1;
}

Answer 2

std::string it's not meant to be used that way. Class allocates its storage sand operator[] cannot expand it, so string cannot become longer. Documented and guaranteed way to append s2 to s1 is

s1 += s2;

Then you shouldn't use std::string because doing that with it is undefined behavior.. Actually, provided length of those initial values, std::string allowed not to use dynamic memory and use class's storage and you have no way to know if it's so from user level of access. It's implementation details.

You have to resize() string before writing in it and hold on idea that size() of string as an index unlike length includes zero character. Resize can change type of storage and allocate storage needed to hold second string. max_size() returns actual amount characters string can store without reallocation.

Answer 3

You can either use += operator or just use insert :

string s1 = "Messi ";
string s2 = "best";

s1.insert(s1.size(), s2);
cout << s1 << endl;
cout << s1[8] << endl; // s

Or with iterators:

s1.insert(s1.cend(), s2.cbegin(), s2.cend());