template<class InputIter, class OutputIter, class Predicate, class T>
constexpr OutputIter
replace_copy_if(InputIter first, InputIter last, OutputIter result,
                Predicate pred, const T& new_value);

template<class ExecutionPolicy, class ForwardIter1,
         class ForwardIter2, class Predicate, class T>
ForwardIter2
replace_copy_if(ExecutionPolicy&& exec, ForwardIter1 first,
                ForwardIter1 last, ForwardIter2 result,
                Predicate pred, const T& new_value);

#include <iostream>
#include <algorithm>
#include <string>
#include <string_view>

int main()
{
    using namespace std;
    using namespace std::string_literals;

    auto s = "hello world!"sv;

    std::string result;
    result.resize(s.size());

    replace_copy_if(std::begin(s), std::end(s), std::begin(result),
                 [](auto ch) {
        return ch == 'h';
    }, 'H');
    std::cout << result << std::endl;
    return 0;
}

Hello world!