Trim whitespace from a String

Question

I know there are several ways to do this in Java and C that are nice, but in C++ I can't seem to find a way to easily implement a string trimming function.

This is what I currently have:

string trim(string& str) {     size_t first = str.find_first_not_of(' ');     size_t last = str.find_last_not_of(' ');     return str.substr(first, (last-first+1)); } 

but whenever I try and call

trim(myString); 

I get the compiler error

/tmp/ccZZKSEq.o: In function `song::Read(std::basic_ifstream<char,  std::char_traits<char> >&, std::basic_ifstream<char, std::char_traits<char> >&, char const*, char const*)': song.cpp:(.text+0x31c): undefined reference to `song::trim(std::string&)' collect2: error: ld returned 1 exit status 

I am trying to find a simple and standard way of trimming leading and trailing whitespace from a string without it taking up 100 lines of code, and I tried using regex, but could not get that to work as well.

I also cannot use Boost.

Answer 1

Your code is fine. What you are seeing is a linker issue.

If you put your code in a single file like this:

#include <iostream> #include <string>  using namespace std;  string trim(const string& str) {     size_t first = str.find_first_not_of(' ');     if (string::npos == first)     {         return str;     }     size_t last = str.find_last_not_of(' ');     return str.substr(first, (last - first + 1)); }  int main() {     string s = "abc ";     cout << trim(s);  } 

then do g++ test.cc and run a.out, you will see it works.

You should check if the file that contains the trim function is included in the link stage of your compilation process.

Answer 2

Here is how you can do it:

std::string & trim(std::string & str) {    return ltrim(rtrim(str)); } 

And the supportive functions are implemeted as:

std::string & ltrim(std::string & str) {   auto it2 =  std::find_if( str.begin() , str.end() , [](char ch){ return !std::isspace<char>(ch , std::locale::classic() ) ; } );   str.erase( str.begin() , it2);   return str;    }  std::string & rtrim(std::string & str) {   auto it1 =  std::find_if( str.rbegin() , str.rend() , [](char ch){ return !std::isspace<char>(ch , std::locale::classic() ) ; } );   str.erase( it1.base() , str.end() );   return str;    } 

And once you've all these in place, you can write this as well:

std::string trim_copy(std::string const & str) {    auto s = str;    return ltrim(rtrim(s)); } 

Try this