c++ operator overload with string comparing - c++

Question

hello i have problem in my school c++ lab, my bool operator > should be return true if lhs is greater than rhs, however it always return false. i try print out lhs.tostring(), it show the number correctly.
my lhs and rhs is a string value.
due to some confidence restrict from my school work, i am not allow to post all the function of my work.
Updated information: ithis lab only can use c++14 and can't include any additional lib. The int value is written in string, and need to compare which is bigger. Assuming there is no negative and any letter other than number
some part of my header file
#include <cstring>
#include <iostream>
namespace CS170
{
class BigNum
{
public:
/* Constructor of BigNum object.
Takes in a character string and
constructs a BigNum */
BigNum(const char * rhs = "0");
/* one of rule of 3 need destructor */
~BigNum();
/* Return a character pointer pointing
to the start of the array representing the big num */
const char * toString() const;
/* Return how many digits the number has */
size_t getNumDigits() const;
BigNum & operator =(const BigNum & rhs);
private:
size_t len;
char* num;
};
}
bool operator >(const CS170::BigNum &lhs, const CS170::BigNum &rhs);
cpp
namespace CS170
{
BigNum::BigNum(const char * rhs )
:len{strlen(rhs)}, num{new char[len+1]}
{
strcpy(num,rhs);
}
BigNum::~BigNum()
{
}
const char * BigNum::toString() const
{
return num;
}
size_t BigNum::getNumDigits() const
{
return len;
}
}
bool operator >(const CS170::BigNum &lhs, const CS170::BigNum &rhs)
{
CS170::BigNum left_value{lhs};
CS170::BigNum right_value{rhs};
std::cout << std::endl;
std::cout << left_value.toString() << " " << right_value.toString() <<
std::endl;
/*this don't work for comparing**/
if(left_value.toString() > right_value.toString())
return true;
else
return false;
}

Answer 1

left_value.toString() > right_value.toString()
This does not do what you think it does. toString() returns a const char*, a pointer to some data. Formally the behaviour of > in your case is undefined since the pointers are not part of the same array, and even if they were, the result would not depend on the string contents.
To check the lexicogrammatical order of strings, you should use the right tool for it, for instance std::string::operator>:
std::string lhs_string{left_value.toString()};
std::string rhs_string{rght_value.toString()};
if (lhs_string > rhs_string)
// ...
// note: here you could simply do return lhs_string > rhs_string;
If you're using a recent compiler and C++17 is an option, you could also use those tools without copying data around:
#include <string_view>
const char* lhs = "programming";
const char* rhs = "language";
std::string_view lhs_string{lhs};
std::string_view rhs_string{rhs};
lhs_string>rhs_string // lexicogrammatical order
live demo

Answer 2

const char* cannot be compared in the way you are trying to. You have to use strcmp. Example usage would look like:
if (strcmp(left_value.toString(), right_value.toString()) > 0)
{
return true;
}
The last part of the function could even be simplified to:
return strcmp(left_value.toString(), right_value.toString()) > 0;

Answer 3

is nearly there but i don't work if compare 11 > 2, as is still read only the first string.
bool operator >(const CS170::BigNum &lhs, const CS170::BigNum &rhs)
{
CS170::BigNum left_data{lhs};
CS170::BigNum right_data{rhs};
int result = strncmp(left_data.toString(), right_data.toString(),20);
return result > 0;
}