#include <algorithm>
#include <iostream>
#include <string>
 
class both_equal_to
{
    char value;
public:
    both_equal_to(char ch) : value(ch) {}
 
    bool operator()(char first, char second) const
    {
        return (first == second) && (first == value);
    }
};
 
int main()
{
    std::string text("_hello___world__");
 
    text.erase(
        std::unique(
            text.begin(),
            text.end(),
            both_equal_to('_')
            // [](char a, char b){ return (a == '_') && (b == '_'); }
        ),
        text.end()
    );
 
    std::cout << text << '\n';
 
    return 0;
}
 

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