Best way to remove white spaces from std::string

Question

I have loaded a text file content to a std::string. And I want to remove the whitespaces from the loaded string.

1. Which below method need to be used for better performance?
2. Which below method can be a best practice.?
3. Else any better way is there to achieve this?

Method 1:
Scan the string using string.find() in a loop statement and remove whitespace using string.erase();

Method 2:
Scan the string using string.find() in a loop statement and copy the non whitespace characters to a new string() variable using string.append();

Method 3:
Scan the string using string.find() in a loop statement and copy the non whitespace characters to a new string(size_t n, char c) variable using string.replace();

Method 4:
Allocate a char* (using malloc[size of the source string])
Scan the string using string.find() in a loop statement and copy the non whitespace characters to the char* variable using strcpy and then strcat();
finally copy the char* to a new string
free char*

Answer 1

Edit: upgraded to locale-aware trait. Thanks user657267 !

Standards algorithms are neat !

s.erase(std::remove_if(
    begin(s), end(s),
    [l = std::locale{}](auto ch) { return std::isspace(ch, l); }
), end(s));

Live on Coliru

That was for in-place modification, if you need to keep the original string however :

std::string s2;
s2.reserve(s.size());
std::remove_copy_if(
    begin(s), end(s),
    std::back_inserter(s2),
    [l = std::locale{}](auto ch) { return std::isspace(ch, l); }
);

Live on Coliru

Answer 2

IMHO, best performance you can get with method 2, but before appending you need to call std::string::reserve method to set capacity of new string to the size of initial string. This needed to prevent unnecessary reallocations, when appending.