C++ Replace string to char[] - c++

Question

I have the following c++ program that multiple 2 large numbers :
#include <iostream>
#include <string>
#define OVERFLOW 2
#define ROW b_len
#define COL a_len+b_len+OVERFLOW
using namespace std;
int getCarry(int num) {
int carry = 0;
if(num>=10) {
while(num!=0) {
carry = num %10;
num = num/10;
}
}
else carry = 0;
return carry;
}
int num(char a) {
return int(a)-48;
}
string mult(string a, string b) {
string ret;
int a_len = a.length();
int b_len = b.length();
int mat[ROW][COL];
for(int i =0; i<ROW; ++i) {
for(int j=0; j<COL; ++j) {
mat[i][j] = 0;
}
}
int carry=0, n,x=a_len-1,y=b_len-1;
for(int i=0; i<ROW; ++i) {
x=a_len-1;
carry = 0;
for(int j=(COL-1)-i; j>=0; --j) {
if((x>=0)&&(y>=0)) {
n = (num(a[x])*num(b[y]))+carry;
mat[i][j] = n%10;
carry = getCarry(n);
}
else if((x>=-1)&&(y>=-1)) mat[i][j] = carry;
x=x-1;
}
y=y-1;
}
carry = 0;
int sum_arr[COL];
for(int i =0; i<COL; ++i) sum_arr[i] = 0;
for(int i=0; i<ROW; ++i) {
for(int j=COL-1; j>=0; --j) {
sum_arr[j] += (mat[i][j]);
}
}
int temp;
for(int i=COL-1; i>=0; --i) {
sum_arr[i] += carry;
temp = sum_arr[i];
sum_arr[i] = sum_arr[i]%10;
carry = getCarry(temp);
}
for(int i=0; i<COL; ++i) {
ret.push_back(char(sum_arr[i]+48));
}
while(ret[0]=='0'){
ret = ret.substr(1,ret.length()-1);
}
return ret;
}
void printhuge(string a) {
cout<<"\n";
for(string::iterator i = a.begin(); i!=a.end(); ++i) {
cout<<*i;
}
}
int main() {
string a,b;
cin>>a>>b;
printhuge(mult(a,b));
return 0;
}
All is working fine, but I need to use char[] instead of "string" . I know it's silly but I have to use that format necessary. So - how can I convert the code to work with char[] definition ?
Any ideas is greatly appreciated, Thanks :)

Answer 1

Provided you don't need to modify the C string (the char array), i. e. it can be const char[] or const char *, use the c_str() method of std::string:
const char *c_string = str.c_str();
Edit: so your problem is that you should not use std::string at all. Well, in this case, this is how you can replace C++ strings with C strings:
C strings are 0-terminated arrays of char (or const char). As usually, in certain conditions, they decay into pointers.
You can get the length of a C string using the strlen() function in <string.h>.
To append strings to each other, use the strcat() or strncat() functions. Beware of buffer sizes and the extra space for the terminating NUL character!
etc.

Answer 2

Call std::string::c_str() on your string objects.
Just make sure the buffer isn't modifed by the functions.
Edit
Or, if you need to accept a char[], just create a string out of it.