What exactly is assigned by the `=` assigment to the new std::string?

Question

I want to write a method that should trim a std::string's ongoing and trailing white spaces. For example, if the method gets " HELLO WORLD ", it should return "HELLO WORLD". Note, that between the return string is a white space, that is important.

This is the signature of my method:

std::string MyHandler::Trim(const std::string& toTrim)

My approach was to copy the 'toTrim' parameter into a non constant copy.

std::string toTrimCopy = toTrim; 

Now I want to get a non constant iterator and erase within a for loop any whitespaces from beginning and the end, while the iterators value is a white space.

for (std::string::iterator it = toTrim.begin(); *it == ' '; it++)
{
    toTrimCopy.erase(it);
}

for (std::string::iterator it = toTrim.end();
     *it == ' ';
     it--)
{
    toTrimCopy.erase(it);
}

This results in the compiler error:

StringHandling.C:60:49: error: conversion from ‘std::basic_string<char>::const_iterator {aka __gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >}’ to non-scalar type ‘std::basic_string<char>::iterator {aka __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >}’ requested

I come from Java and I learn C++ for 3 weeks now. So dont judge me. I suspect, that the = assignment assigns constant char pointer to my new string, so that my copy is implicit a constant value. But I do not exactly know.

By the way, this approach throws an exception as well:

    std::string toTrimCopy = "";
    std::strcpy(toTrimCopy, toTrim);

He says, that he cant convert a string to a char pointer.

Answer 1

It's undefined behaviour to pass an iterator from toTrim to methods of toTrimCopy, so you are lucky that the type mismatch caught it.

std::string::begin has two overloads:

 std::string::iterator std::string::begin();
 std::string::const_iterator std::string::begin() const;

The constness of the string participates in overload resolution. You can't initialise an iterator from a const_iterator, because that would allow you to modify the underlying object through that iterator.

I would change your function to

namespace MyHandlerNS { // optional, could be in global namespace
    std::string Trim(std::string toTrim) {
        for (auto it = toTrim.begin(); it != toTrim.end() && *it == ' ';) {
            it = toTrim.erase(it);
        }

        for (auto it = toTrim.rbegin(); it != toTrim.rend() && *it == ' ';) {
            it = toTrim.erase(it.base());
        }

        return toTrim;
    }
}

Or get rid of the loops entirely with standard algorithms

#include <algorithm>

namespace MyHandlerNS { // optional, could be in global namespace
    std::string Trim(std::string toTrim) {
        auto isSpace = [](char c){ return c == ' '; };

        auto it = std::find_if_not(toTrim.begin(), toTrim.end(), isSpace);
        toTrim.erase(toTrim.begin(), it);

        it = std::find_if_not(toTrim.rbegin(), toTrim.rend(), isSpace).base();
        toTrim.erase(it, toTrim.end());

        return toTrim;
    }
}