Remove all whitespace from a std::string?

[Tyler T]

I want to remove all whitespace from a string. Here's what I have so far:
(expression is the string)

for (int i = 0; i < expression.length(); i++) {
	if (expression[i] == ' ') {
		expression.erase(i, 1);
		i--;
	}
}

But this just removes everything, whitespace or not, starting from the first whitespace. (e.g. If I input "1 + 1" it will give me just "1")
I've tried searching it up on Google and trying the answers. But none seemed to work. This
expression.erase(std::remove(expression.begin(),expression.end(),' '),expression.end());
does the same thing as my code above. And this
expression.erase(remove_if(expression.begin(), expression.end(), isspace), expression.end());
gives me an error (I'm running it in cpp.sh):

213:74: error: no matching function for call to 'remove_if(std::basic_string<char>::iterator, std::basic_string<char>::iterator, <unresolved overloaded function type>)'
213:74: note: candidate is:
In file included from /usr/include/c++/4.9/algorithm:62:0,
                 from 8:
/usr/include/c++/4.9/bits/stl_algo.h:926:5: note: template<class _FIter, class _Predicate> _FIter std::remove_if(_FIter, _FIter, _Predicate)
     remove_if(_ForwardIterator __first, _ForwardIterator __last,
     ^
/usr/include/c++/4.9/bits/stl_algo.h:926:5: note:   template argument deduction/substitution failed:
213:74: note:   couldn't deduce template parameter '_Predicate'

Why isn't my code and the second solution working? Is there any alternative ways to do this? Thank you.

[JLBorges]

#include <iostream>
#include <string>
#include <cctype>
#include <algorithm>

std::string remove_ws( const std::string& str )
{
    std::string str_no_ws ;
    for( char c : str ) if( !std::isspace(c) ) str_no_ws += c ;
    return str_no_ws ;
}

int main()
{
    {
        std::string expression = "I want to remove all whitespace from a string." ;
        expression = remove_ws(expression) ;
        std::cout << expression << '\n' ;
    }

    {
        std::string expression = "I want to remove all spaces from a string." ;
        expression.erase( std::remove( expression.begin(), expression.end(), ' ' ), expression.end() ) ;
        std::cout << expression << '\n' ;
    }

    {
        std::string expression = "I want to remove all whitespace from a string." ;
        expression.erase( std::remove_if( expression.begin(), expression.end(),
                                          [](char c) { return std::isspace(c) ; } ), expression.end() ) ;
        std::cout << expression << '\n' ;
    }

    {
        std::string expression = "I want to remove all whitespace from a string." ;
        int (*fn_isspace)(int) = std::isspace ; // required because std::isspace is overloaded,
                                                // template argument deduction for std::isspace fails
        expression.erase( std::remove_if( expression.begin(), expression.end(), fn_isspace ), expression.end() ) ;
        std::cout << expression << '\n' ;
    }
}

http://coliru.stacked-crooked.com/a/dbfb8120c7d55699

[Tyler T]

Thanks for the help!

[Duthomhas]

You can also use a regular expression, particularly if you intend to apply transformations beyond simple whitespace elimination.

#include <iostream>
#include <regex>
#include <string>

int main()
{
  std::string expression = "I want to remove all whitespace from a string." ;
  std::cout << std::regex_replace( expression, std::regex("\\s+"), "" ) << "\n";
}

Heh...

[Tyler T]

Thanks guys!