Remove whitespace from a string in C++

This post will discuss how to remove whitespace from a string in C++.

By default, the following characters are considered whitespace characters:

space ' '
line feed '\n'
carriage return '\r'
horizontal tab '\t'
form feed '\f'
vertical tab '\v'

1. Using `std::remove_if` function

The standard solution is to use the std::remove_if algorithm to remove whitespace characters from std::string using the Erase-remove idiom technique. Since the std::remove_if algorithm does not actually remove characters from the string but move all non-whitespace characters to the front and returns an iterator pointing to where the end should be. We can then delete the whitespace characters with a call to std::erase.

std::remove_if expects a predicate that determines which characters to remove from the string. There are many ways to specify the predicate that removes whitespace characters:

⮚ `::isspace`

We can use isspace() defined in the cctype header, which checks for whitespace characters classified by the currently installed C locale.

#include <iostream>

#include <string>

#include <cctype>

#include <algorithm>

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(), s.end(), ::isspace), s.end());

std::cout << s;

return 0;

}

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Output:

HelloWorld

⮚ `std::isspace`

Instead of relying on the whitespace characters classified by the C locale, we can use std::isspace defined in header locale, which classify the whitespace character with the specified locale’s ctype facet.

#include <iostream>

#include <string>

#include <algorithm>

#include <locale>

#include <functional>

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(),

s.end(),

std::bind(std::isspace<char>,

std::placeholders::_1,

std::locale::classic()

)),

s.end());

std::cout << s;

return 0;

}

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With C++11, we can use lambdas instead of std::bind:

#include <iostream>

#include <string>

#include <algorithm>

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(), s.end(),

[](char &c) {

return std::isspace<char>(c, std::locale::classic());

}),

s.end());

std::cout << s;

return 0;

}

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⮚ Custom predicate

Instead of using ::isspace or std::isspace, we can even write a custom predicate that returns true if the character is classified as a whitespace character and false otherwise. There are many ways to achieve that in C++:

⮚1. Unary Function

#include <iostream>

#include <string>

#include <algorithm>

bool isSpace(unsigned char c) {

return (c == ' ' || c == '\n' || c == '\r' ||

c == '\t' || c == '\v' || c == '\f');

}

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(), s.end(), isSpace), s.end());

std::cout << s;

return 0;

}

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⮚2. Object of a class implementing `()` operator

#include <iostream>

#include <string>

#include <algorithm>

struct isSpace

{

bool operator()(unsigned c)

{

return (c == ' ' || c == '\n' || c == '\r' ||

c == '\t' || c == '\v' || c == '\f');

}

};

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(), s.end(), isSpace()), s.end());

std::cout << s;

return 0;

}

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⮚3. Lambda

#include <iostream>

#include <string>

#include <algorithm>

int main()

{

std::string s = "Hello \n\nWorld";

s.erase(std::remove_if(s.begin(), s.end(),

[](char c) {

return (c == ' ' || c == '\n' || c == '\r' ||

c == '\t' || c == '\v' || c == '\f');

}),

s.end());

std::cout << s;

return 0;

}

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2. Using `std::regex_replace` function

With C++11, we can also use std::regex_replace for this. This is demonstrated below:

#include <iostream>

#include <string>

#include <regex>

int main()

{

std::string s = "Hello \n\nWorld";

std::regex r("\\s+");

s = std::regex_replace(s, r, "");

std::cout << s;

return 0;

}

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3. Using Boost Library

Finally, we can go with boost::algorithm::erase_all, which removes all the occurrences of the space string from the input.

#include <iostream>

#include <string>

#include <boost/algorithm/string/erase.hpp>

int main()

{

std::string s = "Hello World";

boost::algorithm::erase_all(s, " ");

std::cout << s;

return 0;

}

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That’s all about removing whitespace from a string in C++.

Remove whitespace from a string in C++

1. Using std::remove_if function

⮚ ::isspace

⮚ std::isspace

⮚ Custom predicate

⮚1. Unary Function

⮚2. Object of a class implementing () operator

⮚3. Lambda

2. Using std::regex_replace function

3. Using Boost Library

Share this:

1. Using `std::remove_if` function

⮚ `::isspace`

⮚ `std::isspace`

⮚2. Object of a class implementing `()` operator

2. Using `std::regex_replace` function