How to remove all the occurrences of a char in c++ string

Using copy_if:

#include <string>
#include <iostream>
#include <algorithm>
int main() {
    std::string s1 = "a1a2b3c4a5";
    std::string s2;
    std::copy_if(s1.begin(), s1.end(), std::back_inserter(s2),
         [](char c){ 
                std::string exclude = "a";
                return exclude.find(c) == std::string::npos;}
    );

    std::cout << s2 << '\n';
    return 0;
}

Basically, replace replaces a character with another and '' is not a character. What you're looking for is erase.

See this question which answers the same problem. In your case:

#include <algorithm>
str.erase(std::remove(str.begin(), str.end(), 'a'), str.end());

Or use boost if that's an option for you, like:

#include <boost/algorithm/string.hpp>
boost::erase_all(str, "a");

All of this is well-documented on reference websites. But if you didn't know of these functions, you could easily do this kind of things by hand:

std::string output;
output.reserve(str.size()); // optional, avoids buffer reallocations in the loop
for(size_t i = 0; i < str.size(); ++i)
  if(str[i] != 'a') output += str[i];

Starting with C++20, std::erase() has been added to the standard library, which combines the call to str.erase() and std::remove() into just one function:

std::erase(str, 'a');

The std::erase() function overload acting on strings is defined directly in the <string> header file, so no separate includes are required. Similiar overloads are defined for all the other containers.

The algorithm std::replace works per element on a given sequence (so it replaces elements with different elements, and can not replace it with nothing). But there is no empty character. If you want to remove elements from a sequence, the following elements have to be moved, and std::replace doesn't work like this.

You can try to use std::remove() together with str.erase()1 to achieve this.

str.erase(std::remove(str.begin(), str.end(), 'a'), str.end());