Is there a CompareTo method in C++ similar to Java where you can use > < = operations on a data type

问题

I know that in java there is a compareTo method that you can write in a class that will compare two variables and return a value -1, 1, or 0 signifing greater than, less than, and equal to operations. Is there a way to do this in C++?

Background: Im creating a modified string class in which it takes a string and an arraylist. I want to be able to compare the string in a traditional fashion where if its lower in the alphabet it will be less than, than higher it would be greater than. Than i just want the array list to be linked to the files to store pages in which the word was indexed on in a text file. Anyways the specifics do not matter since i already have the class written. I just need to create compareTo method that would be able to be used in the main of my cpp file or by other data type like various trees for instance.

Ill write the code in java as i know how and maybe someone can help me with C++ Syntax (im required to write in c++ for this project unfortunatly, and i am new to C++)

I will shorten the code to give the rough outline of what im doing than write the compareTo method as i know how in java

class name ModifiedString
Has variables: word , arraylist pagelist
Methods: 
getWord (returns the word associated with the class, i.e its string)
appendPageList (adds page numbers to the array list, this doesnt matter in this question)

Hers how i would do it in java

int compareTo(ModifiedString a){
  if(this.getWord() > a.getWord())
      return 1;
  else if (this.word() < a.getWord())
      return -1;
  else return 0;
}

Then when < , > , or == is used on a ModifiedWord than the operations would be valid.

回答1:

std::string already includes a working overload of operator<, so you can just compare strings directly. Java uses compareTo primarily because the built-in comparison operator produces results that aren't generally useful for strings. Being a lower-level language, Java doesn't support user-defined operator overloads, so it uses compareTo as a band-aid to cover for the inadequacy of the language.

From your description, however, you don't need to deal with any of that directly at all. At least as you've described the problem, you really want is something like:

std::map<std::string, std::vector<int> > page_map;

You'll then read words in from your text file, and insert the page number where each occurs into the page map:

page_map[current_word].push_back(current_page);

Note that I've used std::map above, on the expectation that you may want ordered results (e.g., be able to quickly find all words from age to ale in alphabetical order). If you don't care about ordering, you may want to use std::unordered_map instead.

Edit: here's a simple text cross-reference program that reads a text file (from standard input) and writes out a cross-reference by line number (i.e., each "word", and the numbers of the lines on which that word appeared).

#include <map>
#include <unordered_map>
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <iterator>
#include "infix_iterator.h"

typedef std::map<std::string, std::vector<unsigned> > index;

namespace std {
ostream &operator<<(ostream &os, index::value_type const &i) { 
    os << i.first << ":\t";
    std::copy(i.second.begin(), i.second.end(),
        infix_ostream_iterator<unsigned>(os, ", "));
    return os;
}
}

void add_words(std::string const &line, size_t num, index &i) { 
    std::istringstream is(line);
    std::string temp;

    while (is >> temp)
        i[temp].push_back(num);
}

int main() { 
    index i;
    std::string line;
    size_t line_number = 0;

    while (std::getline(std::cin, line))
        add_words(line, ++line_number, i);

    std::copy(i.begin(), i.end(), 
        std::ostream_iterator<index::value_type>(std::cout, "\n"));
    return 0;
}

If you look at the first typedef (of index), you can change it from map to unordered_map if you want to test a hash table vs. a red-black tree. Note that this interprets "word" pretty loosely -- basically any sequence of non-whitespace characters, so for example, it'll treat example, as a "word" (and it'll be separate from example).

Note that this uses the infix_iterator I've posted elsewhere.

回答2:

There is no standard way in C++ to define an operator that does what the Java compareTo() function does. You can, however, implement

int compareTo(const ModifiedString&, const ModifiedString&);

Another option is to overload the <, <=, >, >=, == and != operators, e.g. by implementing

bool operator<(const ModifiedString&, const ModifiedString&);

回答3:

In C++, you define bool operator< directly, no need to invent funny names, same for operator< and operator==. They're generally implemented as member functions taking one extra argument, the righthand side, but you could also define them as non-member functions taking two arguments.

回答4:

Sun decided to not include operator overloading in Java, so them provided an in-class way (through member functions) to do that job: The equals() and compareTo() functions.

C++ has operator overloading, which allows you to specify the behaviour of the language operators within your own types.

To learn how to overload operators, I suggest you to read this thread: Operator overloading

来源：https://stackoverflow.com/questions/20005392/is-there-a-compareto-method-in-c-similar-to-java-where-you-can-use-opera

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