boost或STL是否具有比较两个char值的谓词？

Question

I need to call boost::trim_left_if on a single char value:

// pseudo code; not tested.
std::string to_trim{"hello"};
char left_char = 'h';
boost::algorithm::trim_left_if(to_trim, /*left_char?*/);

In the last line above, I need a way to pass in the char value. I looked around but I didn't see a generic predicate in Boost or STL to simply compare two arbitrary values. I could use lambdas for this but would prefer the predicate, if one exists.

One thing I want to avoid here is using boost::is_any_of() or any other predicates that would require left_char to be converted to a string.

Answer 1

Since C++11, the idiomatic way to compare for equality to a fixed value is to use a bind-expression with std::equal_to :

boost::algorithm::trim_left_if(to_trim,
    std::bind(std::equal_to<>{}, left_char, std::placeholders::_1));

This uses the transparent predicate std::equal_to<void> (since C++14); in C++11 use std::equal_to<char> .

Prior to C++11 (and, likely, until C++17) you can use std::bind1st in place of std::bind and std::placeholders::_1 .

Remaining within Boost, you could also use boost::algorithm::is_any_of with a unitary range; I find boost::assign::list_of works well:

boost::algorithm::trim_left_if(to_trim,
    boost::algorithm::is_any_of(boost::assign::list_of(left_char)));

Answer 2

Why not just write one?

#include <iostream>
#include <boost/algorithm/string/trim.hpp>

struct Pred
{
    Pred(char ch) : ch_(ch) {}
    bool operator () ( char c ) const { return ch_ == c; }
    char ch_;
};

int main ()
{
    std::string to_trim{"hello"};
    char left_char = 'h';
    boost::algorithm::trim_left_if(to_trim, Pred(left_char));
    std::cout << to_trim << std::endl;
}

Seriously - the stuff in boost is not "sent down from on high", it's written by people like you and me.